Around 20 gauge is good for most body repairs. A piece of VW body is the best, if you only need a bit see if someone is sawing up a bug or bus and cut a chunk out of a flat area.
A cutoff wheel in a die grinder is best for cutting as it won't warp the edges but if all you have is a jigsaw or sawzall then use the finest teeth you can get. Bimetals last longer but cost more and still break off the same when they jam. It's your call.
