I'm looking at a 1994 940 Turbo wagon that is for sale. The owner says it possibly has a burnt valve (according to his mechanic). The car has 175,000 miles. He says this is an easy fix for someone mechanically inclined. I'm wondering if that's true and how much it might cost to fix at an independent shop? The owner has already fixed/replaced all of the other more minor things that could have been the root of the rough idle/low compression in cylinder 3 (plugs, wires, rotor, dist. cap, exhaust, cat. converter). He also says the car still drives fine (he's driven it this way for a few months, apparently), with a slight decrease in power, and that he was told it could work fine for months as is. He says the mechanic ruled out a head gasket as the problem was contained to one cylinder and there was no cross-contamination between oil and coolant, and no external leaks.

I guess I'm wondering if I can trust this information? The car appears to be in excellent condition and well-cared for; he wants $1700, but is negotiable.