1991 Toyota Camry No, power and overhearting in my 1991 Toy

Hello, I am a college student that bought a used car that went kaput six months ago when the water pump went out. I just paid about 200 to get it replaced and the noise went away and it is finally holding coolant. The problem now is, it still runs hot and there is no power what-so-ever. The engine will rev up really high but it will only go 5-10 mph and when it initially moves it will stall from a stop to moving again. The guy that is working on it says what I need to do is replace the thermostat because it was probably damaged when it overheated. We both can tell its not running on all cylinders because we can here ones that arn't firing and when we checked the spark plugs some were wet, and we have yet to look at the 3 under the manifold. All in all do the symptoms sounds like they would be causing so many problems with the engine or could they be bigger problems and more costlier like a fuel pump or something.