I just updated this re: 9mm being equivalent to other pistol rounds w/ modern defensive ammo and no pistol round being a one shot stop...
However, the MechTech looks interesting -- hadn't seen that before. But you'd be in ~$800 for one of those (pistol+upper), which is about the price of the platinum pistol carbine (IMO), the Colt AR, or platinum sub machine gun Action Arms or IMI Uzi.
Honestly, I'd really like the Scorpion EVO, but IF the there was a decent price kit for the VZ2008 conversion, I'd definitely have to consider it.
Re: pistol vs rifle, I wear ~34" sleeve shirts. With pistol + arms extended is over 3 feet/36", probably pushing 40". My sub2k is 29.5" in length (with 16" barrel, most SBRs in 223 have barrel lengths of 1/2 that to have similar length). Just wanted to illustrate that weapon size doesn't correspond to presentation length (and the rifle is easier to maintain control of than a pistol in the event of a struggle IMO).
My preference would be to cut the Keltec down to 10-12" and run it as an SBR. Just not sure that weapon's worth the stamp...