Ok, there are a couple of questions you need to have answered about buying a 91 964. Most important, has it had a top end done? Next, has the dual mass flywheel been replaced?
Early 964's didn't have head gaskets, so a lot of oil/compression leaks. More importantly, all 964's had problems with valve guides wearing out between 50-70k miles. Mine failed at 51k, $6k average for a top end ovhl. Mine also passed smog 2 weeks before failure, failure happened fast, like in one day. Started engine, looking good, 2 hr drive later smoking blue smoke like crazy, used 1 qt of oil in 50 miles.
Early flywheels were a dual mass rubber cored deal, the rubber would break down and there would be no connection between center and outer ring of flywheel, no way to get power to wheels, IIRC.
Does it have the distributor vent kit installed? When? Has the dist belt been replaced? When. The ozone in the dist eats the drive belt, if the belt fails the driven dist stops and sends a spark to one spark plug, it will weld the piston to the cylinder or eat a hole in the piston. Also the car is 15 years old, even with the vent kit it's time to replace the belt. About $300.
When was the 02 sensor replaced, with what? On the 964 they're only good for about 60k, do not buy a generic Bosch, only a Porsche 02 sensor, about $180. The generic Bosch will work, until you wash the car or get caught in the rain. They're not waterproof and will short out the DME killing the engine. After they dry out, they'll work fine, until they get wet again.
I read the listing, didn't say anything about any of the things I mentioned, with the milage listed the work may have been done, if it hasn't, get ready for it. For more info go to;
http://uk.groups.yahoo.com/group/porsche964/Don't get me wrong, I love my RS America 964, but getting her dependable and all the maintenance up to date has cost me approx. $7-8k over the purchase price.
Good Luck, Don