Well, I figured I'd post an update on what I ended up deciding on.
I just bought a '79 Ford 6600, about 4500 hours. Sold new by the dealer I bought it from. Cab, heat, a/c, radio. Needs some work; they just did an a/c compressor on it but the a/c system is still not working correctly, and it appears to be weeping oil out of the head gasket (I'm hoping that it's just an external oil leak).
In any case, it starts easily and sounds healthy, appears to run fine in all gears (at least without a load on it), the clutch and brakes feel fine, it even still had the factory threaded PTO shaft cover on it, so. Overall, I think it's a decent machine.
I would have been much, much happier with something with 4wd because I feel it has a significant impact on the stability of the tractor under certain conditions. But that just wasn't going to happen given what I wanted to spend and how far I was willing to drive to look at stuff.
I should probably start a new thread for this, but, question:
Does the factory cab on the 6600 function as a ROPS? It was my understanding that it does, based on some of my reading, but the machine has no seatbelt and I had someone today very emphatically insist that it is not a ROPS, and the pillars certainly look a little flimsy up close.