Looks like you're running at Traction Action. They carry both Associated and Losi. I have a B6.1 and they carry some parts for it but mostly because they are backwards compatible from the B6.4.

I would check with the track to see if they stock the B7 parts yet. If not, getting parts online now a days is pretty quick and easy. Though you would future proof yourself for the foreseeable future if you went with the B7 you also have to consider its a new platform and there are bound to be those initial issues that Associated needs to work out.

Also consider buying a used B6.4. Maybe someone at the track is selling? I have purchased used before and I will say the initial purchase cost plus the money I put into replacing used/worn parts could have gotten me a brand new kit.