Valve cover gasket wont cause overheating... you just entered a very large area of problems... lol
1. Supra's are notorious (MKIII's) for bad head gaskets
2. 7MGTE's run very hot already... if they overheat.. your gonna have trouble unless its fixed fast
3. the list is long and endless without seeing the car.. but with a MKIII 7MGTE, I always assume a bad headgasket exists. Look for the signs (smell antifreeze in the exhaust, milky oil, coolant scummy, etc...)
At 800.00 if the car is in good shape.. I like it (bodywise), but be aware, unless you are mechanically inclined and have tools, you may be buying a car that wont be drivable in a short while. I would have a mechanic go over the engine first.
READ THIS!
http://www.toyota-supra.info/forums/supra-faq/3296-buying-supra-what-to-know-first.html
It has a lot of good tips.. read the whole thread though.. not simply the first portion.