E46 M3's are the most reliable of all E46's IMO. i would avoid getting a SMG trans, cause repair costs are very high and they seem to be harder on guibos, rear diff/subframe bushings, etc. if you're looking at a 2002-03, make sure the rod bearing recall has been performed. listen for clunking in the rear of the car when test driving, that could be a sign of bad rear diff and/or subframe bushings.

if everything checks out, you're golden.