Re: Stuped questions, sorry
Most LPKs are about the same unless you are looking at one with the lower receiver included. Then you should really look closely at the quality of the lower (Do a search for reviews on line).
Long story, but there are big differences between 5.56 and 223. The easiest way to handle the situation there is make sure you have a barrel that is truly chambered in 5.56 or a Wylde chamber.
The 75 Amax is a long bullet and you will not be able to load it for AR mag length.
It also may or may not stabilize in the 1 in 9 twist. For heavy bullets in that twist you should use the 69 SMK or the 75 MHP that is loaded by Black Hills (my choice). IUt should stabilize well in the 1 in 9.