I wouldn't touch this deal with a 10 foot pole; but that's me.
Here is what I would do if I were you:
#1. Ask if this "new" engine comes with the updated valves that don't fall off into the combustion chamber.
#2. Ask if this "new" engine comes with updated glowplugs that don't fall off into the combustion chamber.
#3. Ask if this "new" engine comes with the 15+ single wheel turbo or if it comes with the fly apart 14 down turbo.
#4. Ask what happen to the old motor.
Get everything in writing. And I do mean everything! Pull an Oasis report and see what other things happened to that truck.
Good luck.