I'm not sure on the Ford deal (I have nothing GOOD to say about Ford), seeing what my Step-Son just went through and is still fighting through on their Powerchoke/Noisestroke Diesel..... Thing is JUNK in my book, Altho, Chevy, has something with the Duramax, (This being said by a MOPAR guy too!) BUT gasoline engines, I'm not with it on Chevy, OR Ford for that matter, speaking Trucks, Dodge ALL the way!
ANY full-size truck, (I'm sorry guys) MUST/SHOULD have a V-8, or Diesel...... As far as Diesel goes, the 5.9L Cummins is a sure way to see 500,000 PLUS, and get good power, and lots of torque, and last with half decent mileage..... IF not the road-worthy 5.9L Cummins, then I'd go with the 5.7L Hemi, in a 1500, but the better choice from what I can see would be the 2500 with the 6.4L Hemi -Stronger truck, heavier, gets not bad mileage, and is almost as strong as the 5.9L Cummins Diesel!
With Dodge however, depending on the truck, 1500, 2500, or 3500, they offer 2 different diesels by Cummins, the 5.9L and the 6.7L. I don't think you can get the 6.7L in anything under a 2500 (I'm guessing here) as thats one strong engine! BUT I know 2017, they're to offer the 5.9L in the 1500...... As a towing package..... Or I read that this is gonna be available!
BUT, for what your looking for, I'm merely mentioning "new" stuff, so if your going to the older Rams..... 5.2L Magnum "318" is a 250,000 mile engine plus. if taken care of, it go a lot longer. and then the 5.9L Magnum "360" is a good one too, but out of the 2 of these? I'd pick the 5.2L Magnum, its dedicated, its worthy, and has a good reputation, and for its power, gets decent mileage. The 5.9L Magnum "360" doesn't get much over the 5.2L and uses a bit more fuel then its little brother the 5.2L..... THEN you have to look and see when the 5.2L Magnum was replaced (I forget) by the 4.7L Magnum..... And I hear good things about that engine as well!
SO in say 2000, I'd find a Ram, 1500, with a 5.2L in it and see how its been maintained, and go from there.....
AND being a truck kinda guy that I am. OLDER Dodge trucks would do well with a Slant 6 in them...... BUT thats up to you..... That is, my ONLY 6 cylinder truck, I'd EVER own! Full-Size trucks with 6 cylinders in the same sentence does NOT belong! -UNLESS its hauling the 6 cylinder.....