I would find out if much more than the governor will need work on the engine. A compression check and oil pressure check will help decide that. Also, see if the radiator looks like it's on its last legs, or if the water pump is leaking when the engine is running and warmed up.
If I remember right, the front end may have to come off to get the governor changed out, but the tractor wouldn't have to be split to do it.
Assuming that all checks out and either you need the loader or the guy won't separate them, then $3000 would be about right for something that appears that could be put right for a few hundred bucks. I'd guess the loader cylinders probably need fresh seals. That, and two new tires and a rim might go something like $250 or so. The governor will be about $80 plus a day or so of putting it in.
If the engine is on its last legs, it's gonna take about $500 worth of parts minimum to go through it, plus your time and trouble if you do it, or somebody else taking several days of shop time. If that's what it looks like, either walk away, or if you think it's what you want anyway, maybe tell him $2000 or so.
I bought a very similar rig, had the guy take off the loader, and later had to go through the engine. Still don't regret it.