Wow that's actually a really good deal if it's in great shape other than the overheating. The overheating could be something quite simple like a thermostat, waterpump, or at worst a bad headgasket. Regardless, I would say it's worth much more than 900 with the low mileage. Perhaps you should first inquire as to what he may have already done to try and fix the overheating problem. If he has already replaced the mentioned parts then it could be a little iffy...