Usually, the series is the first number, followed by the engine size. So, a 325 would be a 3 series with a 2.5 liter engine and a 330 would be a 3 series with a 3.0 liter engine. The letters are simple:
i = fuel injected
C = Coupe
s = sport
x = all-wheel drive
Now, there are exceptions to the rules, like the 323 has a 2.5 liter engine. Which ever care you want to buy, run a search on that specific year and model to find more info.
Are you located on the East Coast, by chance? I know of a 2002 325i auto for sale. Eh, just checking.