Engine size is one thing. Braking has been the one thing no one has mentioned.
I assumed any truck he got would have the towing package with trailer brake controller and "tow/haul" mode on the tranny (uses the tranny to help braking down steep hills) if not then yes, you need to definitely get a brake controller installed. Also like I mentioned an equalizer hitch is a worthwhile investment if you plan on towing much, it will take a lot of load off the rear springs, and make the truck ride much better.
Personally, I would do the Cummins. If you look around and shop right you can easily do a Diesel for less then 16k. As long as the body is in good shape the vehicle will run forever if its a diesel.
back to your OP. I dont know what transmission your lookin for, but deffinitly try finding a Manual. if you can find a 6-speed get it, 5 speed should do it though. Although im not sure if Dodge makes a 1500 with a standard....
Good luck sir!
The Cummins diesel is great, unfortunately you can't get it in a 1500, and the 2500 is really overkill for what he's talking about towing. Also it would be very difficult to find a standard transmission in any late model truck, in the past 7-8 years if you've wanted a new pickup with a manual tranny you've had to special order it.
You don't need a manual though, the newer automatics are just as good or better for towing then a manual, just keep a close eye on the ATF and service it regularly as the fluid will heat up and break down quicker under heavy load. (If the ATF starts to turn brown time to change it pronto) Even many of the newer semi tractors (Volvo, Freightliner, Kenworth) come with automatics now.
PS. For you brand loyalists out there, why even be limited to Dodge, Chevy or Ford. All he needs is a half-ton pickup, and Nissan and Toyota now make excellent half-tons as well. I'm not a brand loyalist, they all make great trucks.
FYIW, my father has an '04 Dodge Cummins 2500, my brother has an '03 Ford F-350 Powerstroke and my workplace had a '05 Chevy 3500 Duramax. I've driven all 3 of them quite a lot, and they all are great trucks. Some are stronger in certain areas then others, but it's mostly a matter of preference. eg, the Powerstroke will do high speed interstate cruising better, but the Cummins will tow up steep grades better, and is more fuel efficient. The Isuzu/Duramax engine in the Chevy is great to, and the Allison tranny in the Chevy is the best transmission, but I don't like the interior of the Chevy. If I were to magically design my perfect diesel pickup it would be a mix of all three... with a Caterpillar engine