History?
Belts replaced? (48k or every 4 years)
Interior, fabric or fabric & leatherette?
Wheels? (15" Original)
Sill condition (they rot from inside out)
Tax & MOT?
Fuel lines ok (they corrode)
Battery tray ok?
Sunroof, if so drain pipes ok, in working order?
Body, paint?
Assuming all ok if mileage is genuine, I would think somewhere around £3-3.5k for a really nice example, the problem is that to achieve that you would need someone in the know to appreciate a series one car, S2's can be had for that money, 944's are in that bracket where a guy can own a Porsche for not a lot of money, then not looked after very quickly fall into an expensive car to put right, then the value as many are now not very good.
Most new to the 944 want a Turbo, again that hinders the value on what is seen as a "lesser" car, the thing is there are not many Lux models about, if it's a good one you may be wise to keep and look after it - you never know?? And they are a great car to drive.