Depends why you are buying it. Sounds like its been sitting awhile and if that engine has never been overhauled its 50+ years old. The G.O. 300 is a geared engine with a fairly low TBO. There was an STC to replace it with a Lycoming O360. So for starters your looking at $30000 or more just for the engine. You can find decent older C172 currently flying for around that. It's is a little larger than a C172 and could be a nice project but probable not worth it if your just looking for something cheap. Get the story on the engine.