I've posted the same at the Hot Roads Forum:
I'm buying,may be in a couple of days, a 1992 500E 67k miles, drives and looks great, but my concern is about the Renntech mods that this car have: Chip (starts in first), exhaust system and (this is the big one) engine heads. About 90 extra HP.
My question is: Is this car going to last and to be as reliable as a "stock" one?

Real tough question. A lot of the "Tuner" parts are very high quality, but people don't try to beef up a performance car with the idea that they are going to not use the increased performance afterwards! If you want max performance and the car is otherwise clean with great records, go for it. If you want it to last and maybe even appreciate in value as a unique car, I would look for one that hasn't been modified.
Some like to wring the max out of a car and figure it was meant to be driven, others enjoy a rare automobile and baby it, never approaching the limits for which it was designed. I don't think there is a "right" answer on this one, more personal philosophy!

JCE, thanks for your reply, I'm going for it; I've never been so excited about driving again a car in my life. As soon as I have pictures I will post them.