Here are my $0.02, and probably not worth that much.
1. Gassers will always be cheaper to own, maintain, and operate. If you are towing less than 10,000 lbs, a gasser will do just fine as long as you can afford the 6 to 8 mpg with the RV hanging off the bumper. The new Ford 7.3L gasser has some pretty awesome towing specs and is getting good reviews, but I am assuming a $75K+ truck is out of the question (even used ones are still bringing $50K in good, low mileage condition).
2. 1983 to 1994 was the IDI, 1994 1/2 to 2003 was the 7.3L Powerstroke HEUI. The engines share nothing expect displacement and manufacturer (International Harvester / Navistar). You definitely are looking at a Powerstroke if it is a 2001.
3. A stock, Naturally Aspirated IDI ('83 to '93) is not a good choice for heavy pulling. It takes alot of mods (turbos, pumps, injectors, cams, etc) and dollars to get an IDI to comfortably pull a 10,000 lb RV down the interstate at 70 mph. A stock Powerstroke is a much better platform to start with if you plan on pulling a RV across the intermountain west.
4. IDI Weak Points. The Glow Plug system is problematic. You will want that system to be in tip-top shape, especially in Montana during the winter (Honey, where's my ether?)
The fuel return lines on the injectors need regular maintenance and replacement.
The Injection Pumps are typically only good for 100,000 miles with todays ultra low sulphur diesel. This is the most expensive part on the motor and can
cost $1000 to $1500 to repair or replace.
5. Powerstroke Weak Points: Keep your oil clean! The fuel injection system relies on engine oil pumped up to very high pressures to fire the injectors. Dirty oil can lead to failed
injectors, HPOP's, stuck IPR valves, and other related issues.
Engine Wiring harness. Wiring harness issues often plague these motors, especially under the valve covers.
Coolant Cavitation: Time to learn what SCR's are and keep that coolant fresh.
6. B10 / B50 Life. The Powerstroke has a B10 (10% of engines will fail) life of 200,000 miles and a B50 (50% of engines will fail) of 350,000 miles. The IDI is generally a bit less with a B50 around 250,000 miles. However the IDI is cheaper to rebuild than the Powerstroke due to the expense and extra complexity of the HEUI system.
7. Drivetrain: Many IDI trucks did not come with overdrive. Ford didn't introduce the E4OD or ZF5 manual until 1989. The early 80's IDI's were backed with a T19 4 spd manual or a C6 auto. Bullet proof for sure, but when backed with a 4.11:1 rear axle you have a truck that tops out at 63 mph. Also, you couldn't get a Dual Rear Wheel with 4x4 prior to the 1998 next generation Super Duty's. Your only option on the earlier OBS trucks was to buy a SRW truck with 4x4 and have an aftermarket company such as Centurion install a conversion kit.
Old diesel trucks are not for the faint of heart or wallet. The red '85 F350 in my avatar was an extremely well taken care of, one-owner truck when I purchased it with only 90,000 miles on the clock in 2016. I have since replaced the IP, installed an overdrive, rebuilt the motor, added a turbo, 3.5" exhaust, new carpet and headliner, new tires, 3 alternators and voltage regulators, new shocks, new kingpins, new brakes, and a bunch of other little things, all beyond the regular maintenance. I am sure I have well beyond double the initial purchase price into those items. In the next couple of months, the fuel tanks will get dropped for new senders, new rear gears are going in, tailgate will be replaced, and possibly an IP upgrade. It is not quite a BOAT, but it is getting close.
Good Luck.
