I am working on my boss' 1991 M5 right, almost perfect balance of handling and power for me. The E60 and F10 M5s he has are powerful but too big, I am coming from an E30 and E90 328xi.
Be wary of parts costs and lots of electronics, like multiple fuse and relay boxes. The older M/S38 stuff isn't getting cheaper, $600+ spark plug wires, $400-500 TPS, $250 crank sensor, $500 brake rotors.
https://fuseandrelay.com/bmw/e34.html
At the same mileage, all the suspension was shot, diff leaks, etc.
I would consider a significant yearly maintenance budget either DIY or indie shop beyond the purchase price.
Any experience with older cars, BMW or otherwise? Maintaining an older car can be difficult if not mentally and financially prepared for their issues.
Do you have a local indie mechanic you trust? The dealer will most likely have no clue what to do with this car, if they do then double the parts prices I listed above plus $180 an hour labor.
Remember is it a 30yo M car, nothing is really going to be cheap or simple on it.