When I saw the M6 in July 2007 it had 2 Cummins engines in it. Also more of the superstructure was intact including the radiators and part of the cab which all now appear to be gone. It would have required a lot of fabrication to restore it in it's previous state and a lot more now. It was $6995 when I saw it, probably less now. I have wanted to buy this for years now, but didn't have anywhere to store a 12' wide heavy tracked vehicle.
They have the track next to the M4 transmissions listed as M47 track but it sure looks like Sherman HVSS track to me. I think the only thing that makes the M6 track special is the center quide but I could be wrong.
As for the M4's, they're quite rusty but of the 40 or so that are there quite a few of them could be restored. Winer's even have Waukesha 145GZ engines for sale as cores.
As for the armour that is parked by the road, there is a Stuart, a Sherman and a Sherman recovery vehicle. From what I have heard, they're not for sale unless you want to buy the whole business.