Other than fit to the upper, problems with lowers are usually a pretty easy fix. Problems with uppers are generally more complicated and not always a cheap or easy fix.

Before I'd go modifying gas blocks and such, I'd get a feel for the rifle. The time to find it doesn't shoot for a hill of beans isn't after you've invested even more in it that what you already have.

I have no complaints about PMAGs, other than they're a little pricey. If you're proficient with mag changes, you don't need to worry about gimmicks like windows that tell you how many rounds are left. The spring and/or the follower are often the culprits when mags fail. Really cheap mags will have sheet metal problems, too. But generally, most GI mags will work just fine, provided you don't leave them loaded with 30 rounds for long periods of time or let them get full of dust and gunk. We didn't have PMAGs when I lived among the Kalashnikovians, GI mags worked just fine for me.