Shotgunner, High Power is a tyipe of competion, nothing to do with the caliber. The Stages are: Offhand (standing) slowfire at 200 yards. Rapid sitting at 200 yards. rapid prone at 300 yards, and slow prone at 600 yards. 20 shots at each stage makes up the 80 shot match.
Ipsc doesn't normally have shots over 200 yards.
Stag is the only one I know of that has left handed ARs, you can put any kind of trigger in it you want. If you use the left handed AR, it would be classed as a match rifle instead of a service rifle for NRA matches, and would not be legal for CMP competitions.
You would need to decide what competion you are going to shoot before building/buying a rifle. ISPC and NRA/CMP High Power are each specailized type of rifle build ups, you would be at a disadvantage or possible ilegal to buy an ISPC type AR and shoot it in NRA/CMP, and vice versa.