I believe that these engines are among the G.M.s that are prone to intake manifold gasket failure. I would likely start with that. Start by doing a pressure test of the cooling system. This isn't expensive or difficult, and will likely show where the leak is. The next test to do is a compression/leak down test, to find out what the engine condition is overall. Depending on the results of these two, you can decide how much the repair is likely to cost, and whether it is worth the time, effort and cost, balanced against how much you love the car/want to keep the car.