Until I stuck a manifold gauge on the system and pulled a vacuum, if possible, it’s very hard Speculating on the cost of an A/C job without knowing the condition of the parts in question, like compressor, hoses, receiver dryer, condenser, evaporator and expansion valve.
If you need to Have a knowledgeable A/C man look at the system to see if everything is there first.
If the compressor is the same as a 1370, which I think they are, those compressor used a Schrader valve. (NAPA sells a 90* adapter for the R12 to R134A conversion; High and Low side)
I can’t think of there true name right now, but they had a tendency to leak with age.
Case offered them at a reasonable price about 2 years ago. I know this because I bought them for my 1370.
Hoses can be made reasonably using a A/C hose crimping tool, buy the pieces you need (*cold hose dot com) or have a hose made (*same place)
As far has cost… it depends on what is there and its condition. I have seen systems put into service for as little as $50 and as high as $1000 plus depending on what is needed.
Normally a new receiver dryer is a must along with an oring conditioner for converting from an R12 to R134A system. (A conversion kit used to be on the market; use NAPA adapters)
If you have to replace the compressor I suggest flushing the entire system and replacing the receiver dryer. That way you can start out with a proper oil charge. A flush kit isn’t that high priced and the flush cleaner can be bought at nat auto dot com
Its been a while but I think an 8oz ester oil is the proper amount for a “COMPLETE” conversion that would include a compressor change to R134A for a case tractor of your size, but you will need to verify that number. Make sure the compressor has oil in it before you start the system.