Does anyone know a few good things to look for in a Carbine? I do intend to shoot it some. Also, I do not quite understand the interchangeability question. Since they were all made to the same specs, parts from one maker should work on a different maker, right?
All USGI parts generally will interchange, so long as they haven't been modified. Anytime you change a bolt, be sure to check headspace before firing.
A few things to look for.....
Most M1s on the market are mixmasters, if you're looking at an Underwood, all original parts will be marked "U something" or "something U". "U" signifying Underwood, and a second character signifying the subcontractor who made the part. The more U coded parts on an Underwood, the more it's worth. All original carbines are very rare, and most matching guns were put together by a collector at some point.
I'm not saying a mixmaster isn't worth having, just use the more non Underwood parts as a bargaining chip.
Matching levels of finish is another thing to look for, i.e. if it's a mixmaster, and all or most of the finish on the parts matches, it probably was a depot rebuld.
That said, look for any un necessary on metal parts. Overly excessive peening or wear on the bolt lugs, slide surfaces, excessive muzzle wear (bullet test should do if you don't have a TE/MW gauge), etc.