MDF will work, but you will need to give it some support. Either transfer the metal banding from the old table to the new one, or glue on some strips of wood/plywood to the underside. This will keep it from sagging. If you want to get real fancy, wrap it with a hardwood, that will protect the edges of the MDF and give you strength.
The corners will take the biggest beating, so put a nice curve on them.
Your sealing method should work, or you could just give it a little spray on poly, or something else easy.