Yes, there are two schools of thought about having formula in the house. I don't think there's a right answer - it really depends on how you feel.
The argument not to have it is that in the middle of the night if you're struggling it can be an "easy fix" and then you just always end up going back to it, whereas if you're always just thinking OK - if I can get through the next few hours - and then you do, it sort of bolsters you to carry on. I think this is particularly true if you think that people around you e.g. partner, mother, etc will be constantly wanting to "fix" breastfeeding by offering a bottle.
Now, I don't actually think it's true that one bottle is going to magically shatter all breastfeeding supply/progress, destroy the baby's gut flora and make them prefer bottles forevermore, which is what it can sometimes sound like breastfeeding advocates are saying - but I do think that what commonly happens is that breastfeeding can be about confidence. If you're already feeling anxious, don't know how much baby is getting, worry that your supply isn't enough or you're doing it wrong, worry that the baby is unsettled etc etc and then you (or DH or MIL or somebody) gives the baby a bottle and it magically seems to solve all the problems - you know they've had X amount, they fall asleep, they seem happy, then this is going to put a huge dent in that confidence which might already be shaky. Then you start to doubt yourself more, give more formula, see this cycle repeat, and before you know it the amount of formula you're giving is enough to affect supply, cause baby to get frustrated at the breast, etc.
In that situation, having no other option but to keep going with breastfeeding (especially when combined with reassurance that you're doing everything right, this is normal newborn behaviour etc) can help build up that confidence and therefore enable you to feel you can keep going with BF.
On the other hand, if you're generally quite confident in how BF is going anyway, you have a bit of knowledge about supply and demand (or sometimes, have little/no knowledge so hence don't worry about it in the first place!) or feel quite relaxed about the idea of mixing and matching, and the intensity of BF being all on you makes you feel trapped and anxious, then having that backup in the cupboard can really help. You might never need to use it but you know that it's there in case you do want to. Or you might use it sporadically to give yourself a break and therefore reduce the pressure and make the whole thing more bearable. Sometimes "all or nothing" is too much and that is absolutely valid. The idea that you must never give a bottle is rooted in an idea that once you start it's always a slippery slope and/or that you can't possibly mixed feed, which isn't true at all.
That said I don't think I will get any in, even though I think I am more likely to be in the second camp. My reason being is that I think DH will lean towards wanting to give a bottle at the first hint of trouble and I don't want the pressure from him. I'm definitely open to giving formula for whatever reason - necessity or convenience or preference - but I don't see a situation where it's absolutely urgent that we need to have some. And if there is one, there are 24 hour garages.
When you buy a pump it will normally come with a bottle or two and if you express regularly, the little breastmilk storage bags work really well. They kind of fold over the bottle and catch the milk, then you seal and write the date on them. They are much easier to store than loads of bottles and you can combine back into the one bottle when you want to feed (it's rare for a single pumping session to yield enough milk for a full feed).
WRT the ingredients in formula, they are very closely monitored and so yes, every single one of them is necessary. The components of breastmilk make up a much much longer list and we don't even know everything that's in it. But all the ingredients of formula serve a purpose and are trying to mimic some function of breastmilk, whether it's a vitamin or nutrient or whether it's the balance of fats/carbs/sugar/etc (I'm not really that clued up on nutrition so not too clear on the differences!)
In terms of food/drink going into breastmilk - breastmilk is basically a blood product, it's made up of white blood cells and fat essentially. Anything that is blood or fat soluble is going into breastmilk, which means most of what you eat/drink is not. Alcohol for example is not fat soluble, but is present in your blood. So your milk will always be as alcoholic as your blood, which sounds a bit alarming but is actually not. If you reached 0.5% blood alcohol percentage, that's a lethal dose - you'd be dead (unless you're a seriously hardcore alcoholic). 0.08% is the legal limit to drive. For context, orange juice contains roughly 0.1 - 0.7% alcohol. On the other hand, THC (cannabis) is extremely soluble in fat so if you take cannabis it will be very concentrated in your milk - should be avoided. Caffeine isn't considered a problem for breastfeeding although some people prefer to avoid it just in case it is keeping their baby awake (if you want to be cautious, just time your tea/coffee after a feed - the caffeine levels peak at about 15-45 minutes after drinking.) If your baby happens to have an allergy to something like cow's milk protein or soya or egg, the proteins from these foods can be found in your blood and therefore will go through to affect the baby, so mums with allergic babies sometimes choose to go on an elimination diet themselves rather than change to hypoallergenic formula. But things like fizzy drinks making the baby gassy, sugar keeping the baby awake, cabbage making them fart etc are probably just old wives' tales as there is no biological basis for this to happen. Tastes from some foods you eat can make their way into breastmilk - this is probably through the fat - although not generally strongly enough to seriously put them off, and sometimes it can be slightly different colours depending on what you've been eating, so expressed milk can look slightly blue, green or creamy/yellow. That's not an indication of the fat content or quality or anything of the sort. It's just slight colouration probably of the fat portion of the milk.