to the forums, Matt.
First, check the FAQ section of this site for a list of common problems with these cars. Then check to see if the car your looking at has maintenance records. Records with BMW cars are VERY important. These cars will last a long time, but only if you maintain them properly.
Have a certified mechanic look at it. It may cost you a half a day and a hundred bucks, but that is a small price to pay to find out if the car is crap.
These cars are lots of fun, as most of us here are in love with our cars, but you want to find one that has been taken care of and have some money left over to do a few immediate repairs on your new purchase (I have never purchased a car that needed nothing, if it didn't, it probably wouldn't be for sale).