buy the best that you can affoard, That doesn't mean the most expensive, just best performing. Zeagle and ScubaPro both have a bunch of afforadable (<$500) regs that consistently get top scores.
Personnally- I have 2 Zeagle DS-V's and they are fantastic, my second stage is a ZX and the octo is the Envoy. I picked them up for about $400 complete each about 3 years ago.
as far as fitment- I don't recall seeing a different size LP port, The LP and HP ports are threaded differently to prevent threading an LP hose into a HP air source, and I have seen 2 different HP port threadings. Basically, if it doesn't want to screw in easily, then you should probably see a tech and get things straight.