I would say do it if the truck is in good shape. The 99-early 03 7.3L are highly desirable. Since it is you uncle, can you ask him to borrow and test pull your TT with it before committing to purchase?
Otherwise, you would be very pleased with an upgrade to a 2007.5 3/4 ton GM pickup with 6.0L and 6-spd trans. You gain about 13% more power, plus a lot more gearing from the 6-spd. You gain 2 towing gears, cruise at lower engine speed, and probably get similar fuel economy. An uprade to this from your current paid for truck should be less than $3k for similar equipment as your half ton.