Aah yay that all sounds brill - love that she is going to check it in her own car for you!!
Baby monitors - I'm a bit old school on this. I have only ever used an audio one. I start using one from when they go upstairs to sleep by themselves before me, which with DS1 was about 8 months, and DS2 was about 3/4 months (I forget exactly). One with lights on it to indicate that there's noise happening is very useful, as it will catch your eye even if you're watching TV or hoovering or listening to something with headphones.
I have used a video one that we borrowed from a friend but personally I don't see the need. It was a pain to set up and the battery doesn't last as long. If I want to see the baby I would rather go up and physically look/check. But I am also not a person who has done sleep training or left them alone to fall asleep by themselves until they are older, so I don't really need to keep an eye on an older/more mobile baby or toddler, or use it to see whether or not the baby is asleep yet without disturbing them, which from what friends say is the useful part of them. DH liked the video.
Those breathing or movement monitors, or you can now get oximeters for babies - I don't like them personally. Reason being. The breathing/movement monitors were originally invented as a way to pre-empt SIDS because it was believed that sleep apnoea related periods of pausing in breathing (which is totally normal in infants because their breathing regulation systems are immature) may be an event which preceded SIDS, and so they are sold on this basis, that if your baby has a non-breathing episode, then they might be about to succumb to SIDS and so by alerting a caregiver, you can go and rouse the baby which sort of "reminds" them to breathe and therefore they won't die of SIDS. But more up to date research shows us that this is not the case. Breathing pause episodes are completely unrelated to SIDS, yet what I don't like is that the companies who sell these monitors still continue to market them as though that initial assumption was correct. And you see it a lot in reviews of them, that people believe the device has saved their baby's life because it has alarmed and then their baby has not died, but in reality the alarm had absolutely nothing to do with it. I find this really unethical marketing, and I don't like it. Because of course if you tell someone "Spend £200 for a lower chance your baby will die" they will do that. You would pay anything to avoid losing your child. And in reality all the monitor can do is provide reassurance that your child has not died, it will not (sadly) prevent them from doing so. If you think you will struggle with anxiety once a baby moves into their own room and want to keep checking them all night even though you know they are probably fine, then it could be worth it on a reassurance basis. But do not buy it thinking that it lowers the chance of SIDS, it does not.
The other problem I have with breathing/movement monitors is that they encourage parents to take more risks than they would otherwise be comfortable with. For example, parents will put infants to sleep on their tummy or move them into another room early, and justify this with "We have a breathing monitor". Now, I'm not saying never go against guidelines, I don't actually think the point of the guidelines is to follow every one perfectly to the letter, BUT you should evaluate risks and make a choice based on whether or not you are comfortable with that risk, and not factor a movement monitor into it, because as I said before, the monitor does not reduce any of the risks, it just lets you know that everything is still fine.
False alarms can also be a problem, and being alerted to harmless (but terrifying!) instances of a baby not breathing or not moving could cause unnecessary anxiety. If you're already prone to anxiety that could even make things worse. All of this taken together, I think these movement monitors are a bit like the topic of using dopplers in pregnancy. Can provide reassurance, can be dangerous in terms of being falsely used as a diagnostic or providing so much reassurance parents ignore other precautionary advice, can relieve anxiety, can cause anxiety through false alarms (!) Perhaps if you think about your feelings on dopplers, you might be able to transfer this to movement monitors?
Oximeters are a slightly different thing, because they are measuring oxygen saturation levels. Now again I have a problem with the marketing of these devices, and they are less like £200 and more like £400. The marketing suggests that sleep is this incredibly dangerous period in a way which is just designed to induce fear and misrepresent the absolutely tiny danger of SIDS. I find this really abhorrent and nasty, a lot of people are already incredibly anxious around SIDS, having marketing ramp this up so that people are more likely to buy your product and you can make money is morally despicable IMO.
OTOH, oximeters are actually used in hospitals, and although they probably won't give you information about whether or not your baby is susceptible to SIDS, oxygen desaturation is a potential issue, particularly if your baby is in a compromised position such as sleeping at an incline (raised/angled surface rather than flat) or has any items around them which could be a suffocation risk. It's quite possible that an oximeter would alert you to desaturation caused by the baby having their face pressed into a cot bumper, or having their airway compressed due to their head/neck position, and since desaturation becomes measurable before it is dangerous, it would give you the chance to adjust their position or remove any items compromising their breathing.
But again you get the same argument - is it safe to adopt riskier sleeping practices such as using a cot bumper or a sleep nest or an angled sleeping space simply because you have a monitor? I'm not sure in this case, because there is evidence that it would actually detect the right problem. But on the other hand it's really unlikely that any direct comparison studies have actually been done, and it might make more sense to just follow regulations like sleeping on the back, no bumpers/toys/nests/pillows in the cot, keep time in inclined surfaces to a max of 2 hours at a time and use the safety harness properly (same advice as car seats), don't use for overnight sleep. And having had a baby in hospital with loads of monitors on them, it was so tempting to rush out and buy every device when we got home so that we could continue to have that reassurance, but in the end I had to tell myself that it was no longer medically advised for my baby to use those monitors and it was sensible to trust the doctors. They are used (for poorly babies) in hospitals, but does that mean they should be used for healthy babies at home? Probably not. If it's medically indicated, you'd be lent one, and if it's not medically indicated - false alarms, dangerous reassurance etc are still issues just like they are for the other products.
About the guidance to have them in the room with you for ALL sleeps until 6 months. I personally take this one with a pinch of salt. I think people can get very het up about this on MN, but I think it's so important to put advice/guidelines into context and make your own choice about what is accpetable to you and what is a reasonable approach to take.
The advice is based on the premise that quite obviously, the risks of SIDS do not magically disappear between 7-10pm and they are not present only for overnight sleep but also for naps. However people tend to see these two types of sleep as "less important" to follow guidelines and so will use things like swings, baby nests, or leave babies alone in a separate room for these periods of sleep whereas they would not do this for the main chunk of overnight sleep which overlaps with your own sleep. But I think you need to weigh it up for yourself. If the benefits of an evening with your partner or an undisturbed room for your baby outweigh the elevated risk for SIDS (which is incredibly small after 4 months anyway) then it might be fine. Likewise if it works for you to spend the evening in the bedroom, or have the baby in the living room, then fine. But I see a lot of posts from FTMs who are diligently doing this, sitting in the dark all evening, missing their partner, feeling like they have no time away from the baby or time to relax and it's causing them PND or a huge amount of stress! For such a tiny reduction in SIDS chances for a very short portion of the night. Most advice that we are given has already weighed up the cost/benefit, and I think this particular one misses the mark for a lot of people. Important ones for me are room sharing until at least 4 months minimum (for the main chunk of sleep), sleep on the back, firm flat surface (cot or similar), no overdressing, no suffocation hazards, no/reduced or distanced smoking as much as poss, and only co-sleep if you're following all safety guidelines. And just be sensible about the rest.