Parts availability is a huge issue with older crawlers. Especially undercarriage parts.
I've got a few small crawlers (under 10,000 lbs.) from Deere, Case, Oliver, and Allis Chalmers.
1010 and 350 Deere. Case 310G. Oliver HG and OC4, and Allis Chalmers HD4.
Parts availability - new and used has been by far the best with the Case and the Deere. But the Case uses more generic parts then the Deere and Deere is phasing out a lot of stuff.
I needed some tracks and rollers recently for my Deere 1010 and Case 310G. To my surprise - new and used parts were much easier to find for the Case.
IH crawlers have very poor parts availability.