Don't waste your $ on re-gearing. Especially if you plan on bumping up the power a bit. It's already a decent HD diesel. It has enough power to haul a camper especially a small one like you're quoting, with ease. Going to 35s isn't a huge jump in tire size. Yes the truck will seem anemic, stock, compared to all the newer diesels, but gearing isn't the answer unless you're really working the truck by towing heavy. There really isn't anything you can put in the bed that the truck will physically haul without severely overloading the suspension and or axles that will slow it to the point of needing to re gear to maintain speed.
By the way, nice find on a super low mile 7.3. They're getting rare
And the several thousand you'd spend on re gearing will go most of the ways towards a built trans when you need it.