I have an e93 and had an e92 and pretty much any car seat will fit. I have 2 kids, 2 and 4 yrs old and have at least 6-7 carseats including 2 recaro's, 2 britax (roundabout and marathon), sunshine kids radian, and graco infant/trvavel system with the snap-in bases.
You will have even more room in the e90 of course, but e92/93 aren't really too bad. The main caveat is with any rear-facing seats, which is what is now recommended for first 2 yrs. These really only fit well just behind passenger seat and preferrably w/o front passenger. Of the ones I've tried, I think you'd probably be happiest with one of the smaller britax or recaro's as these are probably the most versatile for wider weight and age range, and very well-built and safe. You should look for one of the convertible seats that allow you to go from rear to forward facing so you can get the most use out of it. As the kid gets older, the sunshine kids ones are good option because they are low profile and can even fit 3 across in many cars + some fold in half for easier travel.
Also, if possible go to baby store and just ask to try out if you are concerned with fit (or buy from anywhere that will accept returns). I ended up doing a lot of online research and then just ordering online for cheaper prices.