S&W 915 is an economized version of the 5906 but has the aluminum frame. Never handled one but they have a good rep, as do all of the third generation versions of the Model 39/59. I guess it all depends on the price and the condition of the gun you're looking at. And considering price, have you looked at the 5906 and alloy framed 6906? They go for $300-$350 at CDNN (LE trade-in) and other places.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek