Yeah from my experience, focus on service records, if the owner claims they have them, look through them, ensure maintenance items are taken care of such as:
1. - Check the wiper motors on e46 - crazy expensive to fix - part alone is $300-400
1. when the cooling system was refreshed (accs belts, fans, rad, coolant flush)
2. is it still the original clutch and if so how much milage does it have
3. What condition are the shifter bushings in (if your going for a manual)
4. Turn the aux fan to low - and check if it squirlly
5. roll down the windows and check if the exhaust heat shield is cracked
6. Window motors
7. Open the trunk and check for bubbling under the rubber
8. When was the suspension done, ball joints, control arms, bushings, springs, shocks/ struts in that order
That should give you a fairly long list. But be patient...these cars all look amazing when shopping then u buy it and u realize what u truly bought.
Yeah and hold out for a 330 u won't regret it - ull pay a bit more but its worth it.