+1 on that...the weakest bit of the M42 is the cooling system. The water pump and radiator are the weakest parts. BMWs are pretty much like every other car though, just easier to work on and a but more fun. Good luck with the sale.
Definitely get some pics up. This site has problems with *****ded pics, I'd post them at Photobucket, Picasa, etc. and just paste links into your posts. Are you advertising this on R3vlimited too? A lot of us are on both sites...and there's a lot more e30 people over there than there are here.
I'm glad you're right up front about the overheating. What brand of shocks/springs are on the car? What kind of mileage? Sunroof or not (slicktop)? Do you have the original "basket weave" 14" rims, and if so are the center caps all intact? Something like Bilstein Sports and H&R springs will definitely help the car sell.
If the car doesn't have any serious rust and the dash isn't cracked, be sure to mention that as well. These cars are infamous for those problems. I'm sure any of us in the rust belt would love to get a crack at a clean chassis. My car came from Nashville IIRC, with nearly zero rust...that was the primary reason I bought it.
I'll bet you'd get your $2500 if you solved the overheating problem...a new pump is only about $60 online like at PelicanParts or BMA.