One of the biggest things with a crawler is undercarriage. It can drain you financially in a hurry. Everybody sees the pads first, that's one of the least concerns. You would need someone with you who is knowledgeable with crawlers.
If the Td-15 you are looking at is a 1970, then it would be a TD15-B. I understand they were a pretty good machine. If it runs and steers well and transmission is okay, that's the biggest items along with the undercarriage. I would check the oil in the final drives. You don't want a lot of metal, because that's a sign of bearing or gear problems. That also can cost a lot.
Other than that, they are good machines and new, rebuilt and used parts for 15 both B & C series are readily available.
FWD