You might go for any after-market that you would like, basically. You'd just need a harness adapter. If you want to go into the finer points of that option, there are a lot of possible solutions to make the best of it - crossover, amplifier bypass, adding a woofer later on... all on a reasonable budget.
If you go for a replacement BMW head unit, make sure to get one from 1991 or later. The connecting harness and wiring is dramatically different than the first generation head units that went into the E34 (no negative wiring to the speakers... you are reading this right).