A big factor in my tractor hunting was at the distributor level -- namely, has the distributor been in business a long time, and has the ownership been stable? The reason why is that I would be concerned that model X used a 123 transmission, but the follow on distributor only sold model X with a 125 transmission, leaving me with a stepchild and months (if at all possible) parts wait. Primarily a concern because I intend to keep my tractor for 10 or 20 years. In the case of my mid 50's Case VAC, I can still get the important parts.
An example of old manufacturers using others engines? Older Case VACs used Continental engines.
Search and ye shall find whom makes what for whom. OEM relations are important to know as well (such as MF/Agco/Allis/Challenger/later Hesston). Many Korean tractors seem to be an LG of one sort or the other. Many Japanese tractors are Iseki (MF?), Kubota, Yanmar (JD), Shibura and at least one other that escapes me. And the Japanese are sourcing more parts from China.