First of all, congratulations to you and your family. Parenthood has been the most rewarding and inspiring part of my life and I hope that holds true for you.
My son is now 2.5 years old and so I went through a lot of what you're facing. In fact, I sold my special ordered, dream car (a 2008 911S) after my son was a few months old when I realized how unrealistic it was that I'd use that car anytime soon or anywhere near the way I wanted to use it with a baby.
A very small child must be in a rear facing seat (by law). Those, by nature, take up A LOT more space and are often harder to fit in the back of cars. You may find, in fact, that you can not fit a good rear facing seat to fit in your E92. Since your rear seats are not flat, you're going to need to use the towel or pool noodle method to level the seat. I know a lot of people use towels, but I had very good luck buying a cheap pool noodle and cutting it down. It retained it shape better than a towel, didn't slide, and absorbs most of the bite of the plastic from the base of a car seat. You're going to have to do a bit of trial and error based on what car seat you buy.
There are lots of models of seat protectors which can help protect the inside of your car. I have protectors made by Diono (
http://us.diono.com/en/car-seat-accessories/super-mat) in all three of my cars and they seem to be doing an outstanding job of protecting my leather while making sure I have access to the LATCH system and not slipping. Britax and several other companies make similar products.
Also, check with your local and/or state authorities... here in CA the Highway Patrol will help you install your seat and/or check to make sure your seat is fitted properly. There's a disturbing statistic that says that something like 80% of car seats aren't installed properly and as such, can do more damage than good. I know I had to make an appointment a few weeks a head of time but it was invaluable to me, especially the first time I put car seats in my cars making sure everything was safe and installed correctly.
Finally, don't cheap out on a car seat. You can get seats for very cheap but just as I always tell people who are considering riding a motorcycle, buy the very best safety equipment you can find. I'm on my second round of car seats for three cars (as your child grows, he/she won't fit in the same seat and eventually he/she will move to a booster) and I've dropped a lot on car seats. Thank God I've never had an accident with my son in the car but I'm glad I haven't cut corners.
I hope this helps but again, most important of all... congrat's. Health to you and your about to be expanded family.
PS - get some sleep... you're gonna need it.