As I just traded a 2011 3500 Ram SRW w/ Cummins for my 2017 Ram 1500 I'll give you my opinion FWIW.
My 2011 was a decent truck, much nicer than the 1995 2500 5-speed w/ Cummins I sold before getting the 2011. The few times I towed with it, it performed flawlessly. Most of the time, it was driven empty. If mileage is a concern, my 2011 got 15-16 mpg on the hi-way. If speeds were kept below 70, it could eke out 16.5, maybe 17 provided the route wasn't too hilly.
I did have it deleted for 4 years, but the mileage didn't increase appreciably. I've read where folks have claimed incredible mileage gains after deleting; good for them, I didn't see it. Driveability was a lot better, but I didn't delete for HP/torque gains; only to get rid of the hated EGR and exhaust after-treatment (DPF, etc). I returned the vehicle to stock once inspection stations here began looking for all emissions equipment, and ran it stock again for a year before deciding to trade. (with full disclosure to the dealer)
I made the move to a 1500 simply because I really didn't need a 3500, and frankly not a 2500 either. And I was getting frustrated with diesel in general.
The truck you described should have had it's oil separator filter (above the valve cover) replaced @ 64k miles. Not good if it hasn't been done, and will need to be done ASAP. The EGR cooler and associated plumbing also needed to be cleaned @ that 64K mark, and is quite the PITA, I've done it. If it hasn't been done on this truck, another issue. I religiously replaced the fuel filters every 10K miles. The service history for that will need to be determined. The ATF and filters in the 68RFE trans should have been changed every 30K miles. Another question to ask. The cummins equipped 4x4's tend to wear ball joints, some before the mileage of the truck in which you are interested.
You mentioned wanting to delete at some point. Since the environmental punishment agency began cracking down on diesel tuners a few years ago, as far as I know, all the tuning boxes for these trucks have been pulled from the market. You would have to find one second hand. I used an H&S mini-max, but wound up selling it to the person who purchased my truck from the dealer to whom I traded it.
Another consideration about deleting is there has been quite a bit of discussion about deleting, and ultimately having a head gasket failure. This is largely dependent on which tuner (as different methods were used to generate the higher hp/torque), and right foot management, and even with that, installing a good quality active waste-gate to keep boost in check as well as monitoring exhaust back pressure are strongly recommended.
If you don't delete, the DPF will likely be in need of replacement ~ the 150K mile mark. A new DPF unfortunately isn't cheap.
Another consideration is injectors on some of the 6.7 Cummins can fail. They don't just stop working, they wind up injecting too much fuel, and you will notice that the engine is "making" oil. If that happens, the oil will have to be tested to determine % fuel dilution, and then each injector tested. Not every Dodge dealer is capable of doing this work BTW. Another thing to keep in mind.
Just a few things to consider. Sorry for the long post.
