I have a batch one AR45, havent got it built yet though. It uses standard AR15 parts, with the exception of the mag catch which is proprietary.
I would reccomend holding off on it. Since Justin doesnt have a manufacturer's license, we had to buy the 10% lowers from him, and Mitchell at Khalan Weaponry finished the lowers for us.
There have been numerous about complaints about Mitch. Lack of communication being one of them. Out of spec lowers being another.
Bazooka Bros also has a lower to use GG mags, however a mag catch slot has to be milled into the mag body (one comes with the lower, IIRC). It looks like a GG magwell bolted to an AR15 lower. It also uses oly style uppers. If you cant mill the slot for the mag catch on new mags, a local machine shop or gunsmith should be able to for a small fee.
The oly .45 carbine uses 41AE UZI mags, which are hard to find and expensive when you do. I think oly is looking to make their own mags, or have them made for them, but dont know if the ball ever got rolling.
If not for Mitch at KW, the AR45 would be the best thing going in terms of a .45ACP AR.
You have a few options:
1) Get a 10% from Justin and finish it yourself, if you're able to do so.
2) Wait untill Justin and Bill find someone else to finish their lowers, I know they've talked to a company or two about this, but havent heard anything solid, so I'll leave names out.
3) Get a 10% now, and have it finished later.
4) Get the BAZ45 lower.
5) Use the plastic Cav Arms lower and the Hahn block to use GG mags.
If you want one soon, the BAZ45 lower looks like its going to be the best option, I dont know the current legal staus of Cav Arms so I dont know how easy their lowers are to come by. Personally I dont care much for a plastic lower that doesnt offer any options as to the grip and stock.
ETA: More info here:
http://ar15.com/forums/topic.html?b=3&f=15&t=345390