I'll add my voice to the 6bt Cummins fans. My rusty old D350 has 280k miles now, still runs perfect, doesn't burn or leak a drop of oil, and gets 23 mpg highway. It is a big, heavy engine, but not way heavier than an old Ford 6 gasser.
The 4 bt would also make a nice job for you, but they are a little harder to find. Rusted out first-gen Dodge/Cummins are all over Craigs List in the Northeast, easy to find and cheap to buy.