I don't think any manual tranny is hard to rebuild, if you can get it all apart. I rebuilt one many moons ago in a D-50, and only needed bearings, syncros, thrust plates, and seals, and I think I spent about $100 plus lube. I don't remember having to have anything pressed apart/back together, which is the main stumbling block. All my bearings drove out of the case with a brass drift and BFH, and I used no special tools. There are some upgrades in the thrust plates and I think in the input shaft bearings available for later models that retro fit into the older case. I'd change the pilot bearing in the crank tail, too. Weak point of these trannies seems to be the input shaft assembly. I've seen dozens with a really floppy input shaft, on the order of 1" nose of the shaft deflection.

I've also seen a couple of 300k mile trannies that were tight as new (did need new syncros), but almost all of these had been run with atf in the box instead of gear lube. Draw your own conclusions.