Hi, Nim!
Welcome to the wonderful world of black powder cartridge shooting!
In answer to your question about what barrel length for hunting and target shooting, it's personal preference! Many shooters like the extra two inches of barrel sight radius and extra weight at the muzzle. Others don't mind extra iron to carry in the field, but prefer the muzzle heavy feel for offhand. Still others (me included) like the Mathew Quigley image and WANT the longer barrels!
Be assured that for the .45-70, all your powder will be burned in either 30 or 32 inches of barrel. What you need to consider is that the total maximum weight of your Sharps 1874 will vary a bit according to the wood density as well as the barrel weight. BPCR competition has maximum weight limits.
A "top punch" refers to the bullet sizing top punch that correctly fits the BPCR bullet you use. Almost all top competitors shoot custom cast or have cast for them, the high quality bullets they use.
For more online info, try doing a search for "black powder cartridge rifle", or "BPCR competition". There are several good online sites that share ino and have chat rooms!
HTH
John