Make sure the water pump impeller has been replaced with a metal one.
Check the radiator, particularly the plastic neck.
The rear shock mounts are known to fail. If they do the car will sound like you have a door open when you go over bumps.
The HVAC system can fail.
Some cars have had vanos faliures, but that seems to be rare.
The automatics are not particularly robust, but you should be looking at manual cars . . .
I cannot stress how important a pre-purchase inspection from a competent mechanic (INDY BMW specialist or dealer) is.
I don't mean to discourage you, but it sounds like you are stretching your budget to buy this car, and I doubt you have an extra $100-200 a month to spare on maintenance costs. And these costs don't spread themselves out over each month, but rather are off and on, i.e. nothing for 8 months and then a $1200 bill.
Buying a used high performance car is not the same thing as buying a new car of similar cost. You need to factor in the extra maintenance costs.
I had a 325i that was $240/month for the note and another $100/month in maintenance/repairs over the time I owned it. If you are going to modify the car, that will also cost. I spent about $100/month on modifications while I owned the car (2.5 years, 50k miles).