You get what you pay for.
On swapping from dual wheels to single, why would you do that if you're wanting something that can handle weight like a class 5? I understand you have width restrictions, but you're also cutting down the max weight you can carry by doing that. Just how much modification are you wanting to do to your rig? And how long are your trips going to be?
BTW, Cummins engines are NOT a Dodge only engine. They manufacture engines for all applications. Dodge just shouts "CUMMINS" engine so much because they can't make a decent engine for ******%, so they have to outsource for an engine. You can power a ford with just about anything out there. If you're wanting to save some cash, you can buy an F550 with a burned out engine and drop in a diesel of your preference easily.
Honestly, it just all depends on what you'll be doing.