Unique powder is pretty much a "one size fits all" for handguns.
Get a dial caliper as opposed to a digital. Dial calipers don't rely on a battery so it'll never die. Digital isn't any more accurate than a dial. You really can't measure to 4 places with ANY caliper. Calipers are only close...
Don't overlook Windham Weaponry. It's the old Bushmaster crew in the old Bushmaster building owned by the original Bushmaster owner (RIP Richard ****). It's a top quality firearm and it fits your budget.
One can NOT get "M1 Thumb" when loading a FULL clip of 8 rounds--PERIOD. I've got 45 years of trigger time behind the M1 rifle, left-handed at that, and have NEVER ONCE had M1 thumb. Not even close.