Thanks for the feedback @RSIno1 and @NikA.
I was under the impression that the thickness of the barrel had to match the barrel channel in the stock. I'd read that the 1A barrel is too thin to thread. Likewise, the barrel would need to be thicker than I'd like in order to fit in the barrel channel of a 1S or 1H. Or one would have to fill in the barrel channel to get a thinner barrel to fit in the 1S or 1H. Sounds like I may be worrying over nothing and I can pick any style of No 1 that strikes my fancy? Or perhaps one could make any barrel thickness fit in any barrel channel if you have the money.
As far as extractor compatibility, I should look for the 308 family, 30-06 family, or 7x57 family? Anything greater than 223 and smaller than the magnums? Perhaps it's the same here as the paragraph above - one could pick any cartridge you'd like, it'll just take more time (read: $$$) to convert extractors and other bits from a much smaller or larger case head.
@NikA - I'm not planning to hunt subsonic. Yes, there will still be a supersonic crack, but I'm just looking to get the decibels down under 130. I'm hunting elk and mule deer, and very occasionally pronghorn.
Again, thanks for the initial feedback @RSIno1 and @NikA. Learning a lot from you both.