First off, common items that I've had to replace on my 2000 328i prior to 100K:
-standard maintenance such as spark plugs, air filters, fluids, brake pads and sensors, bulbs, etc. ($150/yr on average)
-camshaft sensors (intake and exhaust, $100 each)
-brake rotors ($50 per wheel)
-rubber air intake duct ($50)
-final stage unit (a/c component) ($70)
All told, I budget at least $400 a year in maintenance on my car (add $400 if you don't diy). That tends to cover the cost of 4 new tires one year, followed by fluids another year, followed by brake system replacement the next, repeat...
And I disagree with Zibby. There have been reports of the AT malfunctioning on these types of trannies, but it is not a guarantee. People commonly complained about slowly losing power in their Reverse gear before the tranny finally lost the gear. Statistically, of the several (if not tens of) thousand of 3-series produced from 1999-2001, very few have reported this problem. Take good care of the transmission, don't butcher it with your driving style, and it should be just fine.
I'd also recommend taking the car to a dealership/mechanic for the following tasks (or diy):
-compression test the cylinders
-check for error codes (AFTER the test drive)
-check subframe for damage at mounting points
-check color of all fluids, compare to new
-check under seats for trash/growth (water damage could result in fungal growths, stiff carpets)
You didn't say what the person was asking for it, but if they haven't replaced the camshaft sensors, maintained all of the fluids, or replaced the brake components in awhile, you might ask them to drop the price accordingly or just pass on it.