Best advice I can give you is to get a PPI (pre-purchase inspection) done by a qualified P car shop. That way you know what issues it may have and what items have been recently replaced. You can use this site to help you find one near you
Independent Porsche Repair.
Looks to be pretty clean. Just needs a good headlight restoration. (kit
HERE if needed). It also seems to be a very well traveled 996. I'd also recommend checking your local "for sale" boards (like craigs list, cars.com, auto trader, truecar.com etc) in your area to see what these are going for with mileage and the condition close to this one. This should give you a good idea on what a fair price would be for your region. Good luck with the purchase.