It is a bit of a minefield OP.
Infant car seats are all the same, yep, that about sums it up. I am a bit of a car seat enthusiast (
- how sad is that?) and I completely agree with that comment. You basically can't go wrong with them because they are such a simple design and the fundamental part of that design - that it's rear facing - makes them incredibly safe. As mindutopia says, when you're looking at the seat they will go into after the infant one, then you need to be much more picky and careful, because for example some very cheap ones and even some quite popular ones, if you look at the crash safety, they are absolutely dire. There are some companies which are really bad for it. But if you look at the infant carriers of even these companies, they are fine. They score just as well for safety as seats from the most highly respected brands.
So I would not get too worried about the infant seat.
As for features:
Spinning you don't need with this stage, because the spin feature is useful when you're leaving the seat in the car and installing the child into the seat in the car. Whereas with an infant carrier, usually you will strap the baby into the seat in the house, and then carry the seat to the car. So it doesn't matter what direction you put it into the car, because you can just...turn it around with your arms :)
Isofix base - eh. It's slightly quicker than doing it with the seatbelt, but personally I don't think it's worth £100+ which is what most of them cost. DH reckons it was. I say, save it for when you have DC2 and you feel guilty about not buying them anything new, you'll actually want the extra 10 seconds it saves you then, because you'll also have a toddler to wrangle. Isofix vs seatbelt doesn't make a difference to safety except that a wrongly installed seat is unsafe, and isofix makes it pretty much foolproof to install. But seatbelt install really isn't that hard. Watch a youtube video (all the companies have them) and decide if it looks complicated enough to be worth it. (Joie i-level is one of the rare seats that won't install without a base. In that case it's included in the purchase of the seat, though.)
Isofix base that also takes the next type of seat - Almost always overpriced, massively limits your next seat choice, and if you have another child within 3 years, you may not even use this function. If you do get this, look further than this base's compatible seats when you upgrade. It's not always the cheapest/best option.
i-size compatible. If your car is i-size compatible, probably worth it right now, simply because it eliminates the need for a fitting. If your car isn't i-size compatible, don't worry about this. It's a slightly higher safety standard, but most old infant seats would have passed it anyway. If you do get an i-size seat, they come with height limits, whereas old ones don't. I recommend a height limit over 80cm as opposed to a lower one. It gives you a better overlap with the next type of seat.
Side impact protection - if you're interested in safety I would actually recommend looking for seats which state they have this. All the brands have their own various ways to achieve it. Also, all i-size seats have enhanced side impact protection. Will protect your child in the event you get T-boned. Some older, more basic, and/or non-i-size seats do not have this. That's the main difference in safety for infant carriers when looking at i-size and not.
Weight of the seat - worth bearing in mind. The average newborn baby weighs between 3-4kg on top of the weight of the seat. You carry the seat one handed. By 6 months this is more like 7-8kg and by 9-12 months, if you've gone for a heavy seat in the first place your arms will feel like they are coming out of their sockets. Add the weight of the seat to the weight of the baby. Joie i-Level is a very heavy seat, BTW.
Lie-flat seats - Potentially a bit of a gimmick, however I have heard people who absolutely LOVE them. Lie-flat position in the car is potentially thought to be a bit less crash-safe than the normal 45 (ish) degree angle. But obviously the 45 degree angle is potentially dangerous in terms of newborn breathing, so... something you simply have to weigh up. This is mainly an issue in the first 4 weeks. After that the recommendation is they can be in the seat up to 2 hours. It might be useful if you plan to bring the sleeping baby into the house in the seat - personally, I did this anyway, and just kept an eye on them. If you're really concerned about it the term to research is "positional asphyxiation in car seats".
Insert or padding or seat design to "make" the seat curve flatter for newborns - IME seems to actually work a bit better than the seats that market themselves excessively as lie-flat. However, tricky to judge without seeing a real newborn in the seat, you will not be able to do this unless you get very lucky with a photo search or review online.
Sunshades - useful, but you need one on the car window as well unless it's tinted.
Seat that "grows with your child" and covers older ages - not always terrible, but choose very carefully as often these types of seats make a compromise somewhere. Biggest issue is you won't be able to take it out of the car and carry it around, which is a bit of a pain. Again, consider future siblings. Best if fitted professionally into your car to check angle - not possible at the moment.
Compatability with a pushchair/travel system - Most brands are only compatible with their own type - Silver cross with Silver cross, Graco with Graco, etc. However. There is a sort of "universal" car seat attachment. This is commonly referred to as Maxi-Cosi adapters because Maxi-Cosi was the first brand to use it. If you buy an infant carrier seat from:
Axkid/Besafe/Cybex/Joie/Maxi-Cosi/Nuna/Recaro/Venicci
And/or your pushchair states that it's compatible with one of these brands, you can use any infant car seat from the above brands except Joie Juva (that only goes onto 2-3 Joie specific prams). Also, Cosatto Dock is Maxi-Cosi adaptor compatible, but the other Cosatto seats aren't.
The other oddity is Britax - some Britax pushchairs can be bought with a Maxi-Cosi adaptor, and any pushchair which is also popular in the US will have the option of Britax Click & Go adaptors meaning you can put a Britax Babysafe on there.
Special covers - Some claim to be "breathable", some you can buy a special summer cover to help wick away sweat in the summer months, some are made out of pure lamb's wool so that they don't contain any chemicals. If you get a popular brand and end up needing new covers you can generally find them second hand. Don't ever buy/use covers made by anyone other than the company who produced the car seat, though.