I've dinked around with the concept of an AR pistol, but I haven't done it yet (I still have 4 receivers left out in the shop). Something about that buffer tube just looks kinda gay, like a big dildo sticking out towards my face or something.
On the other hand I have 4 AK pistols, 3 AMDs 1 Yugo Krinkov and plans to cut a romanian kit into a pistol. I feel they are great defensive weapons, I've shot the krinkov from the modified weaver stance with far better accuracy than my 8" ruger redhawk.
If you do buy/build an AR pistol just use the CAR length tube, as the old pistol tube was unreliable. I see the 7 1/2" barrels are prone to needing tinkering to run 100% as well (from what I hear 11 1/2" is the only length than runs very well out of the box.)
My only gripe with the AR platform is the buffer tube though.
I see where you're comeing from with the CCW thing, you just want to keep it in your car, right? Luckly in my state you can carry loaded long guns conceiled even if you don't have a CHL.
No, I wouldn't buy a SBR, most people will tell you never to use a NFA weapon as a defensive weapon, it should be kept in a safe.