Sounds like a good deal. Its very hard to find a running driving truck in decent shape for even $3k.
The gas gauge is more than likely the sending unit in the tank. If you replace it your better off doing the entire fuel pump assembly with the sending unit.
The AC could be anything but the battery.
Heater hose leak? maybe. My guess would be either the bypass hose or T-stat gasket, a water pump is also a possibility.
Ticking noise could be a leaky exhaust manifold. they tick when cold and goes away when warm. If its in the engine it will be a bit harder to diagnose.