I just bought one myself (link to pictures here in these forums
) and am VERY happy with the experience thus far. Let me tell you things I was looking for.
1) I absolutely positively would not buy from anyone but a BMW dealer, and the car MUST be BMW pre-owned certified at that. Most of the time it entitles you to a 100,000 bumper to bumper warranty.
2) Take it out on a lengthy test drive. Get it way up in speed and listen for any noises or shakes. In addition, take some normal, slow, and high speed corners. Feel for anything out of the ordinary
3) Make them clean/detail the car top to bottom, inside to out before making a decision. Take 15 minutes to look everything over thoroughly. Check the leather to ensure there are no cracks or tears, look for any dings/scratches in the paint. Look for any damage that might have been caused by an unreported or low speed wreck.
4) Most importantly (and directly tied to my #1 point), look at the service records. If you purchase it from a quality BMW dealer, and it's pre-owned and certified, and it's around the 2002 range with 30,000 miles as you stated it was probably on lease through them. They should have 100% of the service records for the vehicle and should be more than willing to show them to you. I literally spent 20 minutes going over every detail in my new car's service records and found the only issues were a flat tire, and a low speed curb incident that required some additional work. I know it got the appropriate oil changes and checkups.
On price, just to give you a heads up, just hit KBB.org. Expect maybe a little bit of a discount off of that depending on any deals that are up, or how desperate the dealer/salesman might be. Mine is pretty loaded (premium audio, sport package, premium package, heated seats, etc) with a KBB of about 30,500. I walked away about 28,000 very very happy with a car that you would have a hard time telling wasn't brand new.
Just my two thoughts as I just got done with that exact process.