A 1-9 twist will properly stabilize 55 and 62gr rounds which constitute about 90% of the offerings for the AR-15 to digest. If a heavier grain round is desired for hunting, I'd reconsider the AR-15 platform all together, and based on the OP's questions, I seriously doubt he'll be needing something along the lines of 77gr SMK's with a 1-7 barrel and throated chamber.
Bottom line, is a 1-9 twist will treat the average person just fine.
The biggest questions to consider when purchasing an AR-15 is A2 vs A3/A4 receiver, chrome lined vs not chrome lined, profile of the barrel and then finally 16" or 20" (with a multitude of others, but that builds up the primary lengths)
My recommendation is if you're looking for a gun to just do some plinking, maybe occasionally trying to hone in on accuracy, to look for an A3/A4 style AR-15 with a 1-9 twist chrome lined bbl, HBAR profile, and 16".
The velocity out of a 16" is sufficient for anything sub 250 meters when firing at paper, the chrome lined will allow for less strict cleaning practices and accommodate the heat generated by anyone wanting to just touch off a mag, HBAR to provide solid barrel harmonics for accuracy and weapon weight, then of course the A3/A4 flat top to allow for an easy transition between a scope, holo-sight, red dot or iron sites.
I'd recommend learning iron sites from the start to understand the fundamentals of the AR-15 platform and to learn how diverse POA(point of aim) is from POI (point of impact) due to the elevated sight height above barrel.
EDIT:: Had to add some gratuitous gun ****
