Change your oil. Is it foamy and milky looking?
Drain and fill your radiator. Does it have oil in it?
If either of those is true, your head gasket is blown. You only get white smoke if the head cracks in an odd way and you get coolant into the exhaust side. Everything the first mechanic charged you for most likely means it was the radiator (all the little stuff like hoses is standard). The second mechanic might have been "shooting in the dark" about the water pump. I say that because the first guy seems to have done a nice radiator job, I doubt he would have missed a leaking water pump. (pretty much the second thing you check)
The head gasket sounds likely at this point. Its one of the reasons I dread overheating so much... once is enough sometimes. You have a new water pump, radiator, hoses, and thermostat... they have replaced your entire cooling system