Things to look for when shopping...

Hey guys, i'm looking for a nice daily driver. Something fun, RWD, fairly reliable, and must get at least 20mpg avg.
I've been looking at late 90s 328s, but i noticed that i can get an M3 of the same year for about the same price. So hell might as well do that right?
So questions...
What kinda mileage would i be looking at for a '99 M3?
What are some things to look for when shopping?
What are the problem areas with these cars?
Also, of the non-M3 3-series's, which would you recommend? 323/325/328?
They changed the body style in '00...any major mechanical improvements?
And can someone give me a quick rundown of the BMW trim levels? I get that the last 2 digits are the engine size and the first is the model, but what are the meanings of the "i" vs the "Ci", etc.

I know, alot of questions, im sorry. Completely new to the Bimmer world :icon16:

thanks for your help guys!

Greg