Jason,
The sprocket looks well past it's time and you will be very lucky to get by without throughing a track.
39 acres of clearing can mean allot of different things, especially in what part of Texas you are in. I spent two months running my Case 1550 Dozer and a full sized backhoe cleaing 7 acres. I had my dad helping, and we took out every single tree to build my lake.
I don't know how much power or how heavy an old D6 is, but mine is just under a newer D6 in power at 169hp. I cannot just push over any old tree. I can push over most trees a foot or so thick, and sometimes even bigger if it's been raining and not too hot out. Bigger trees require digging out allot of dirt, buiding a ramp and pushing them over. This can take hours for one tree. I've found that a backhoe will take out a big tree so much faster then my dozer that it's not even comparable. If I can't push over a tree in one or two attempts, then I leave it and come back later with the backhoe.
How much clearing do you plan on doing, and how thick is it? Thinking that you can do half or even a quarter as much clearing as a guy in a new machine who knows what he's doing isn't. realistic. If the guy who gave you a bid said he could do it in one month, I'd figure six months at the soonest to do the same thing. Probably twice that.
The hard parts isn't even taking out the trees. That's easy. The hard part is getting rid of them. Moving hundreds of tones of trees and brush takes allot of skill with your blade and machine. If you only end up with one yard of dirt every time you push a pile of debri to the burn pile, you'll be lucky. My dad has hundreds of hours on my dozer, and he gets tired real fast and loses concentration. The next thing you know, he has a 20 yard pile of dirt in front of the burn pile. Dirt doesn't burn, so it's more work all around.
I agree you can't lose for that price, but you should still be ready to spend some time wrenching on it and givng up some of your cash. Dozers love lots of cash!!!!!
Eddie