[moxie]

Bryan,
A couple things to keep in mind. First, .45 ammo is expensive. It's about triple the cost of 9mm. So practice and plinking are going to be expensive. Second, most .45 carbines available today are expensive and not necessarily the most reliable guns around. As noted above, the Hi-Point "goes bang just about every time you pull the trigger." Not exactly what I want in a defense gun. The Olympic Arms upper looks interesting. Don't know how reliable it is. But notice mags are $45.44 dollars each. Ouch! And I don't know how reliable they might be.
If you can find an affordable 9mm carbine that would be the way to go, but I don't know of a real good option being produced right now.
If you really need/want a carbine the best option, IMO, is an M-4 style AR-15 in 5.56/.223. Can be had in the $700 price range, although supplies are tight right now. They are as reliable as a carbine can get. Mags work and are cheap. Ammo is available and cheap. One more caliber won't hurt. And, the .223/5.56 round gives you amazing flexibility you can't get with .45 or 9mm.