Considering you already know part of the final drive is bad, you could about expect the rest of it to be a money pit also. If you plan to get something that's reliable enough to do real work, I think you'll need to be looking for something newer/in better shape.
I think you hit the nail on the head here. I bought a JD1026r TLB and have been doing work for people in really tight spots. I wanted a cheap dozer for a big job that I have coming up because I figured I could buy an old one for about twice the cost of a rental. I think i'll just do the rental and not have to fool with rebuilding the whole undercarriage. Otherwise i'll find someone else that has already done all the work for me and I'll just get an older rebuilt dozer for around $5k once my truck is paid off. That would be the cheapest and fastest way I think, since they will never get back their labor costs.