Undercarriage parts are getting scarce for many crawlers. So, if you buy - I suggest you either make sure the undecarriage is in excellent shape, or make sure you can still get replacement parts. Aftermarket companies like Berco have stopped making many part for older crawlers.
I've got several small crawlers at different properties. Deere 1010 and 350, Allis Chalmers HD4 and HD6, Oliver HG, OC3 and OC4, and Case 310G.
Besides parts availability, ask yourself if you want a foot clutch or hand clutch, loader or dozer, and if ease of repair an issue. Some are easy to work on and some very complicated.
Also - having a hydraulic reverser is real nice but - it also means a huge expense if repair is ever needed.
How about the drive? Clutch driven or torque converter? My Allis HD4 is torque converter drive which personally do not like. Parts for AC crawlers are also getting very hard to find.
I think the easiest crawler to work on is Case. 310 or 350. No steering clutches to get bad and most parts out in the open. Good parts availability also. Better then for most older Deere crawlers. May be too small for what you want? A Case dozer (6 way blade) weighs around 8000 lbs. My 1010 crawler-loader weighs 10,000 lbs.