Check out this link for some tips.
This is the king of obsoletesí website. He hangs out on the repower website sometime. They have a good section on construction equipment and some very knowledgeable people on the old crawlers. Here is my TD6 that I built out of scraps of 2 junk machines. It is running working machine and is worth 2500 on a good day, it is getting harder and harder to find parts for because they are worth more as scrap then they are as a running machines. I guess people figure why put the effort into getting one going that has been sitting just to get the same money as what the scrap man will give you. It is worth more to me than what the market will pay for it. .