i, personally, have no experience with any DPMS product, but i've heard that 5.56/.223 7" barrels might have some problems ejecting or cycling, but again i have no personal experience with this.
i did a Form 1 on a stripped lower and im waiting for the paperwork to come back. 1 SBR lower can have an unlimited amount of short barrels and calibers on it, as long as the registered length is kept on the gun when stored.
i also think building AR's are somewhat easy and fun. ill post pics under the NFA section when my paperwork comes back (hopefully soon).