I bought a BCM bolt and put it into my Bushmaster. Instead of buying an entire new carrier as well, I used an automatic center punch to stake the keys. Just go slowly and move the metal to ensure that it is contacting the screws.
I wouldn't pay that much for that particular model. It's got an Izzy brake, which is supposed to be horrendous. Plus, it has a fixed carry handle, so it will limit your options if you want to put optics on it. My Bushy is a flat top upper with removeable carry handle and a YHM Phantom brake installed courtesy of Zero Hour arms. The Phantom is a nice low cost brake that isn't nearly as bad as the ones that come on most stock MA-compliant AR15s (miculek, mini-Y comp, izzy, etc).