1999 Isuzu Rodeo Question Valves

I have a little white smoke from the exhaust pipe and when the radiator cap is off with the engine running the water bubbles. Someone told me that means I have a bad valve. Also 3 of the new plugs are black on the ends. It overheated once and it was only a matter of about a block maybe 2 blocks that I drove it and noticed the needle on hot. No smoke, noise or anything at the time. Also there was what appeared to be a metal gasket that knocked my timing belt off so it wouldn't start. Mechanic took it apart and found the piece of metal and a bolt. The metal has chewed up marks from the timing belt. He told me there was possible valve problems but he didn't think so. He "said" he tested it for head or head gasket problems and there was none, but possible valve problem. He knows I don'thave money to repair it so he said possible and didn't want to keep my car to further investigate. Is it a bad valve and how much money are we looking at?

Thanks for your time.
Sherry