Lower construction is not too crucial. Sticking with good brands that are known to be in spec is the only consideration. DPMS, RRA, Spikes lowers are all to spec and won't be a problem.
The issue with running a reliable set up depends on your gas and buffer system. While some might think a 7 inch barrel would be a ball of fun (ball of fire too) you would be correct, however the gas systems for consistent reliability are found on longer barrels and is one of the reasons I went with an 11.5" BCM upper. Take dwell time into consideration when choosing your barrel length.
The buffer is the second variable in the equation for reliability. The older pistol tubes are shorter systems than carbine and M16 length buffer systems. RRA and Spikes both make carbine length pistol buffer tubes and when coupled with a carbine length gas system improve reliability considerably and permit access to readily available parts and easier solutions to cyclic problems. And you can get an actual cheek weld on the tube giving a consistent sight picture with that of an AR-15 rather than holding the pistol tight against a sling for stability.
9mm vs 45 ACP? The answer is 42.