Nothing really wrong with the plastic guide, millions of rounds have been fired through countless Glocks using them and they rarely break. For that reason I haven't researched aftermarket RSAs. Sorry, I don't know which weight spring GM uses.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek