There have been all sorts of problems with this motor. First and foremost the early ones (up to '94) i believe, were designed with an alloy in the cylinder walls that was damaged by the sulfur content of US fuels. Basically the rings ground into the cylinder wall, causing at first a rough lopy idle (almost as if a bigger cam was installed) and if left untended almost complete loss of compression.
BMW did replace a lot of these engines under factory warranty and under a good faith warranty, I have seen several with replaced motors.
Secondly the 91 you mention is most likely "Badge Engineering" as some have put it, since until '93 it was 735i / il or 750 il. The 3.5 motor is actually far more reliable, but a bit sluggish in those cars. But it should be on par with a 300SE.
Lastly, the cars just dont last. The interior wears, falls apart, suspension, transmission and mechanicals shouldnt be an issue at 75K, but expect expensive repairs as you cross the century mark.
My advice, for the 13k that you want to spend - I would seek out a newer, younger 300SE, or sel, maybe a 91. The V8's are expensive to maintain, and the M103 has proven itself as a motor, (well except for the valve seal issue) but that is a relatively minor expense. The W126 chassis is timeless, and becoming extremely reasonably priced.
By all means however, stay away from an early W140. They are money pits by now.

Best of luck in your car search,
George Androulakis