Well this can count for the two of them I've had. I owned identical 535i 5 speeds, '88 and '86, with about the same mileage to start. I put 25,000 miles on the first car, then 25,000 miles on the second, each within a little over a year.
I'll include prices with labor to the best of my recollection:
'86 Starting mileage: 149,000 Ending mileage: 175,000
Starter went out: $220
Power antenna: never fixed
Power locks went crazy: never fixed
Shift linkage (shifter came off into my hand) : $120 (fixed myself)
Three batteries (alternator was OK, but drains?), batteries were replaced free under warranty: $85
Pedal bracket (requires master cylinder too) : $700
Ignition wires: $100.
'88 Starting mileage: 121,000 Ending mileage: 149,000.
Engine crank position sensor (commonly mistaken as a dead ECU) : $225
Pedal bracket (yes, on BOTH cars) : $600
Bad heater blower: Never replaced
Blown headgasket: DEATH.
Haha, where do I begin? I enjoyed the first one so much that I found a second one just like it. Granted, I drove the **** out of these cars... so hard that I have been called by friends as "the destroyer of cars".
What I don't understand is besides regular oil changes and the repairs above, these are NOT expensive cars, these are NOT complicated cars, these are NOT unreliable cars... especially for being 20 years old or more now. GREAT examples can be found for less than $3000.
The first one I had got entered directly into BMWCCA events and got awards... I found the thing on a used car lot in amazing condition for $4k. The second car I got for $2000... the poor guy took great care of it and said no one wanted to drive the thing.
Why are these so undervalued? The e30s don't hold a candle to these things motor-wise... the m30 has a s**tload of torque. These things can drift too, let me tell you. And with a few mods, my first one could take e39 drivers on a regular basis... needless to say, they were shocked. With some weight out, they come in just over 3,000 lbs. You get that, leather, power everything, and a moonroof.
One thing I must say you should do immediately is go with some Bilstein heavy dutys... it will transform the mushy suspension into a better handling car than the factory e28 M5.
For fun, I had taken the entire exhaust off (including cat) and run straight pipes for the track (yes, I tracked both of these cars). This is by far one of the best sounding cars at full tilt I've ever heard... there is motorsport hiding in that ordinary sedan.
These cars should not fall into the hands of pussy-footed "car collectors"... drive these things hard and you will be rewarded with the seeing the soul of an otherwise forgotten "old" car.