If it's a 5.56 SBR lower, a standard blowback upper is the cheapest option, but will require a buffer swap, too. Just the buffer - the carbine spring is fine. This is by far the most "universal" buffer I've found for any 9mm upper:
https://kakindustry.com/9mm-carbine-buffer-extended-gold-topThe FM blowback stuff isn't really crap - they recently updated their barrel feed cones so they feed better than before. It's not as good as the Mat-9, Macon Armory, or Aero/BA EPC feed cone, but it's an improvement. The only things IMHO that should be discarded immediately are their buffer and recoil spring. I bought one of their stripped aluminum lowers as a test platform and it runs well.
I have a Matador Mat-9 upper I'll be testing soon. Reports are that they work, AND you don't need to worry about swapping buffers when you swap uppers since it's all one self-contained unit. Their feed cone (funnel shaped barrel face bevel that leads into the chamber) is excellent and should feed practically anything.
Building an RDB upper is a good choice, too. As Amphibian mentioned, they use the ejectorless endomags since the ejector is in the bolt. Again, no buffer swap needed.
...and Mean arms has their new Exomags on pre-order (I have one on order for testing) that's a complete mag without needing to install a kit in a Pmag.