No easy answer here, everything depends on how much of a rebuld you want to do,
Engine Rebuild Kit 600.00 - 750.00
Machanic labor 65.00-75.00 per hr.
Machine shop work ....... i.e.
Cylinder head 100.00 w/no cracks to weld
Engine Block 250.00 w/no cracks to weld
Crank shaft journals 100.00 w/out balancing
Hydraulic pump rebuild kit 150.00
If the tractor actually needs an engine rebuild, then it needs alot more than TLC, in fact, such would be a major investment, dollar wise, as is shown above in parts and labor.
What about the elctronics What about the carburetor
What about the steering
What about the transmission
You really need to have a tractor machanic look the tractor over before you make any decisions about purchasing.I don't believe anyone can give you a dollar amount as to it's worth, without hearing the tractor run under working condions.
You may not need but only a portion of the above, to be performed to get the tractor in good operating condition, then again, you may need even more. Remember, a second hand tractor is a problem that it's owner no longer wants to deal with.
Good Luck .......... Scott