When I was shopping used trucks the issue I ran into was the Cummins and Duramax both held their value too well to be worth buying used. Basically I'd be trading $8-10k for 80k+ miles and several years of use, which wasn't worth it to me compared to buying new with a full warranty. My hope is that I keep this truck a long time.
One option I considered but ultimately opted against was buying a used 6.0/6.4 Powerstroke and then spending a few thousand to bulletproof it. From what I found the Powerstrokes, especially pre-6.7, really depreciated a lot faster so the prices were lower. There's a reason for that of course but it's an option to consider.
I guess one other thing to consider is the emissions stuff. On the earlier generation diesels they didn't seem to have things sorted out too well compared to the new ones, so there could be issues to deal with there unless you live in an area where have the option of deleting.
Check out cumminsforum.com for some feedback from Ram owners.