You can convert an existing .45 (glock/1911) pistol to a carbine with a Mechtech full top end conversion, or in the case of the 1911 (and I think Glock), an add on stock, and 16" bbl for use with your existing slide.
As far as a new gun is concerned, There are things I would, and would not bother to buy. The price of the weapon weighed against it's usefulness
doesn't make sense for the HK USC ($1500), or an AR platform ($7-900). Unless I had an AR15 Lower already, I wouldn't bother with a pistol caliber AR.
Unless I'm feeling nostalgic, the Thompson doesn't catch my attention.
I would recommend you look into the Beretta CX4 (best overall situation)
as it is a quality firearm with good modern features.
A marlin Camp Carbine used would also be worth looking into.