The SOS transmission is not that hard to work on IF you are familiar with modern car automatics or tractor power shift transmissions. It just is planetary and band that are hydraulically applied.
They where fine IF you did not do stupid things with them. Under heavy loads don't shift with out clutching when you shift the gears that shift two clutch packs at the same time. That was a large jump that would jerk the tractor. They would do what ever you told it to do. So if you wanted to shift to road gear chained to tree it would try it.
As far as cost the parts are getting high priced and many are not available through Ford anymore. A lot of the discontinued parts where just orings and such. These you can make or buy. Some of the valves are in short supply too but many times you can have yours repaired by some one that has a machine shop.
It sounds like it would not lose you money even if the trans was bad. The sheet metal,tires, and engine will bring more than the $1500 they are asking. I would not push it too hard on the price. If anyone that is knowledgeable about Fords finds out about it they will snap it up. The diesel engines are in demand. The transmission parts are worth a lot too.
How I would go about it is to put the engine back in the tractor. Then get it running. With it running you then can tell if the engine is good and if the transmission is good. This is the best way to tell if problems are in the SOS. I have had SOS trans apart that looked fine and they would not work. Had to go through a shift/trouble shooting chart to locate the problem.
I just bought a 981 last year with a gas engine that had a broken piston in it. The tires are 90% but the sheet metal is rough and the rims are mismatched. I gave $1000 for it. So to me the $1500 for a diesel even apart is not too far out of range.
The things I would consider: 1)Are you wanting a tractor to use a lot or just one to collect use a little? 2) How good of a mechanic are you?
3) How big of a project can you complete?
If you are wanting a heavy usage tractor then I would pass on it. If you are just an average mechanic then I would pass on it. If you have trouble completing long projects then I would pass on it.
You can get a running Gear drive model for $2500-3000 so unless you want something rarer to collect then I would pass it.