Picking the right truck

The '90's Chevy 6.5L diesel was very underpowered and not that reliable. The '01 - '04.5 LB7 Duramax has problematic injectors. If you want a Chevy diesel I would stay with a '04.5 LLY or newer.
The Ford 7.3 was a great engine but they normally command high prices even today. The 6.0 is very problematic and should be avoided unless you are prepared to spend thousands of dollars fixing all the problems with them. Also, resale will be quite low. The Ford 6.4 had a reputation as a fuel hog and is even more expensive to fix than the 6.0.
The Cummins 5.9 was pretty much solid no matter what. The problem with them was the poor 4 speed automatic transmission that so many were equipped with. If you can find a 5.9L with a manual it would be best.

They are out there.. chevy ford you may need to go above 20k But stick with 2007 ram year and up with 6 speed or 5 speed auto and they are in the 20k range.. Look for 150k miles or less.

Just my opinion..

This has the cummins 6.7 6 speed manual though

https://www.carsforsale.com/vehicle/details/46271996

Sure glad we have our 2006 dodge dually with 75k that is in mint condition. We would be hard pressed to give it up. Good luck on your search.

The Ford 6.0 has a bad rep but as you noted they can be fixed and there are a lot of 6.0 owners that never had any of the problems. That being said I'd still avoid them if you can find something else in your price range.

Diesels hold their value, that's good if you own one, not so good for you. You will also find that duallys are rare beasts on the used market. You may have to search far away from where you live. I'm sure you are aware of the dangers of EBay when buying vehicles. If not be very very careful. You certainly want to take whatever you find into a diesel mechanic to look at before signing the paperwork. I'd be looking for an older Ram. Or and older Ford 7.3

Your looking for an old truck then. Or one that is a strippo. Heck my 07 Dodge dually 4x2 whosaled for 24,500. And I only had 42K miles on it

Good luck with that. In today's market getting a used dually for 20 grand in good condition will probably be next to impossible in as much as new ones are upwards of 80 grand today.