Quoted:
Gotch ya. I've been thinking about this for a few years and as such I've been reading up on the legalities associated with it, at least on an NFA/constructive intent level.
I'm going to buy it all as one so I don't have to worry about buying the upper before the lower (I want to insure a good fit from the manufacturer.
So you understand - don't do anything blatantly dumb.
I have a Spikes lower and Adco built 10.5 upper - carbine length gas and buffer worked out well. Just like anything else, stick to good stuff and you should be fine - really depends on what you (the owner) wants. My lessons learned: Pay attention to the bolt / extractor (definitely a place to spend an extra $20) and beware the lure of a shorty buffer.....
Have fun, shooting a fire-breathing AR pistol has been known to cause uncontrollable smiles.