If the headgasket went bad, you would have some sort of oil-water mixing. Pull the dipstick and look at the oil, if it looks like chocolate milk, since the headgasket was replaced, you will have to re-seal the oil cooler attached to the oil filter mount. Sometimes, a PITA but very doable w/ a little/lot of patience.
Engine overheating, try this, burp the system. Where the top radiator hose goes into the engine, there is a bleeder screw. Unscrew the bleeder screw and squeeze the upper hose, close the bleeder screw. If you get air, the system wasn't bled properly and this could easily cause your overheating problems.
If no air, the thermostat is probably jammed. Bigger PITA.
Look at
www.clarks-garage.com and click on workshop manual. All of the processes above and MANY more are described there in detail.
Unless it's real cheap, find another example. there are plenty of well maintained examples in Florida to choose from. It doesn't depend on your budget cause if you get a cheap 944 w/ lots of problems, you'll spend twice as muchmoney fixing it up than if you bought a nicer, better maintained one.