This FAQ well answer a lot of your questions.
http://www.ar15.com/forums/topic.html?b=3&f=62&t=378850
The magazine compatibility and cost is a non issue for me, Pmags run $20 and ArmaLite GenII's are $25. The older Gen I ArmaLite mags were counterfeited a lot, so there are a lot of substandard early mags. If it's build on a USGI body with a ArmaLite marked bottom plate you are gtg.
IMHO, Noveske makes the most accurate rifles, and ArmaLite makes the most robust rifles. ArmaLite is the only manufacturer to redesign the bolt stop and magazine catch for the AR10 platform. The others have retained the AR15 parts.
There's a lot of nice stuff on the used market at very good price points.