I would be suspect... why did the seller not reprime the tractor? I would take the time to inspect and reprime the engine before purchasing. If he baulks at the idea then more might be wrong. I have had good luck with CL but I am careful. Good luck!!
None. Maybe a dead battery from cranking it, worst case.
Reprime: Open/bleed/close each bleeder screw in sequence: 1) fuel filter inlet, 2) outlet, 3) then the one down on the injector pump inlet. No need to bleed beyond the injector pump.
Like Mully said if the seller won't cooperate - run away. It's not tractor, its a lawn ornament.