Yow, the E31 8 Series. If you want "fast" then the only one your want is the 850CSi which came out in either 94 or 95. With 372hps/402 ft-lb torque, its the one to get. However, expect to pay.
Good condition ones with all service records, and you want ALL service records, will sell for between $45K to 60K and up - and that's assuming you can find one!
Note, do a google search for the 2 part E31 Buyer's Guide by Paul Micheal Brown. He is the 8 Series guru and will tell you all the pluses and minuses. Big minus - 2 of everything, complex electronics, expect to budget a minimum of $1K-2K per month for maintenance (if not more!). Good Luck!