I would say 2 gallons would be about right. You may have a quart or so left over but that is always good to have around down the road for any touch ups or mishaps. One gallon of clear will yield two gallons when mixed with reducer if 1:1 mixed. That should be enough to clear it with three coats. Primer depends on the condition of the metal work. If it is pretty straight then you could get it prepped with a gallon of sandable primer but I like to use a couple coats of epoxy primer to start and then reduce epoxy primer to use as a sealer before shooting color. Really no use for thinner except to clean up the equipment as your paint and primers will come with the appropriate reducers for your particular situation. If you haven't found your supplies yet there are some good places online that have very good quality stuff. SPI products sells a really good series of clears and primers and Chad's paint supply out of Michigan sells some really good base colors and they are both about half the price of the big name stuff. Your painter should have a good idea on the amounts he needs as he has done this before. Also he may have a specific brand that he is used to and likes. Ken