Not an easy answer as your search is very broad. I'll try to give you some good pointers....
-Engine type is personal preference. If you do well over 15K per year, diesel is a good option. You can get either in a 2500. 1 ton's are almost all diesel. As stated, you do not need a dually at this time. If you plan to upgrade in the future, consider it an option.
- Most suspension systems in the rear are leaf spring type unless you look at 14'+ Rams. These have a coil over 5-link rear suspension, ride & tow like a dream. Leaf springs are fine, but do ride rougher and are prone to having broken leaf springs.
- Regardless of what you choose, inspect the under carriage carefully. Are any components in need of replacing? How are the shocks? Is the bed and frame in acceptable condition to receive and aftermarket hitch? How is the electrical system?
- Short bed vs. long bed is again personal preference. There are hitch systems for both.
- Try to get something that has an integrated exhaust brake. Not a mandatory item but is well worth having to help in downhill speed control. You'll also need a brake controller.
I am biased towards Ram and a firm believer in diesel power. But that's my preference. At a minimum you need a 2500 that will not leave you stranded.
2014 Ram 2500HD CTD MegaCab-Big Horn Edition
2016 ST Reserve 26RB