Just an opinion from a Kalashnikov fan, relatively ignorant of the AR ways...
Personally, I don't see very much need for the AK brake on an AR. Recoil and muzzle rise is not an issue with the .223. The brake increases the noise and may have a cool factor look if you are into it. Personally, I think the extra 1.5" of barrel (or so) could be better used to milk a little more velocity out of the .223 if you are going for a shorty Bushy.
#2... for a little more cash, you could get a NIB Bushmaster that will truly be "yours" from the getgo. Not sure if the used one will be covered by warranty, but the NIB one will. Might luck out with a decent trigger to boot.
Again, I could be way off base as ARs are not (yet?) my thing. Just an opinion that you should NOT go for this deal.