tranny and tuning advice

So I just got my first BMW, a 94 325i, owned by a little old lady. She had all the maintnance records from day one and is in near perfect condition. Heres my question for all you BMW sages out there. First of all, twice now the transmission module warning light has come on at startup. So, I shut it off and wait a few seconds and restart the car and like magic the light stays off and away I go. Is this module really expensive, and where is it at? Ive looked through about 700 threads in this forum and didnt see anyone else having this problem. Im not really worried yet since the tranny shifts fine the rest of the time. Second, what mods are the best for this car? What intake and exhaust combos are good? Is this car sensitive to extensive mods ie: turbo or supercharger? has anyone put underdrive pulleys on all the accessories, how about a chip? Will 19" rims fit or do I settle for 18's? And finally, whats the difference between the E30's, E36's, E40's and so on? Everyones help is greatly appreciated.

u can fit 19's in the back, but only 18's in front