"...Factory M2 Ball is 147 Grain..." A 147 is a 7.62 NATO bullet weight. M2 ammo, not that you'll find any, uses 150's, now, at 2800 fps. W.W. II vintage M2 ball used a 152 grain bullet. There's no M2 ball readily available, up here, anyway.
"...older Military Ball is likely at least mildly corrosive right up to really corrosive..." French milsurp is corrosive for sure, no U.S. made M2 after 1952ish, Korean 'KA' stamped is, but not 'PS'.
"...use the lightest weight bullets I can find?..." No. A 150, 168 or 175 grain bullet. The latter are match bullet$.
Federal makes M1 Rifle specific ammo. 150 grain FMJ's and BTFMJ',s. They also make an M1 load using a Sierra 168 grain Matchking.
Hornady loads a 168 grain match bullet. Do not use any 'Light Magnum'.
"...look for in a Garand..." Over all condition. Dents in the stock are no big deal, they can be steamed out(steaming doesn't fix cut wood fibres), but look at the handguards for cracks and you don't want a grease/oil blackened stock either. Field strip it, if you can, looking for rust and/or pitting under the stock. Barrel should be shiney with crisp rifling. If you have one milsurp cartridge, you can do the "bullet test". Put the bullet into the muzzle, the closer the case comes to the muzzle, the worse the muzzle wear. This is a field expedient test.
Wiggle the gas cylinder. Tight is good. Ditto for the rear sight. The sight should have definite clicks when the elevation and windage knobs are turned. The front hand guard should be tight as well. The whole rifle should be tight in the stock. Trigger guard should 'firm up' close to closing. The whole trigger group should be tight in the rifle too.
Look around for some clips while you're at the show. You'll be needing them. Don't pay more than $2 each though.
