As with anything used, if it was maintained correctly it should be fine. Reoccurring issues I've read about are water pump failures (assuming you are after a Cummins powered truck and related to 2013 and 2014 production) and transmission issues (68RFE only, no issues with the Aisin) if it was overly lugged in sixth gear while towing heavy. I have a 2015 and zero issues except for the current SCR recall.
As far as emission issues, first, these newer Cummins trucks (2013+) are tuned for performance, except for an EGR (greatly reduced usage) all emissions are handled downstream in the exhaust system with a DPF and SCR system. There is a real mess going on, RAM and Cummins suing each other over the costs of a recall and a class action by RAM owners trying to get RAM and Cummins to provide a solution. You can read more about that SNAFU at;
Ram Diesel Owners Suing FCA, Cummins Over Inflated Fuel Economy | Off-Road.com Blog
My thoughts on deleting, not until I have an emission issue that will cost more to fix than to delete. Deleting doesn't do anything for engine performance since emissions are handled downstream in the exhaust system. Think about the human health issues NOX creates (turns to acid in our lungs). I have grandkids and I don't want them or anyone else breathing my NOX, leaving my emissions system intact keeps NOX out of their lungs. Take a read through this thread;
http://www.ramforum.com/f158/another_upgrade_ctd-106869/
There are several options for tuning without deleting (Google EFI Live), EGR only deletes, and of course full delete tuning. My facet is, with 800 pounds of torque in my truck, and how well it pulls my RV I have zero need to increase engine power. And when you do increase power, you will quickly find the weakest link in the drive line. A full delete, tune, and transmission bullet-proofing are going to cost somewhere in the neighborhood of $7,000 for a 68RFE truck, there doesn't seem to be any tuning available for the Aisin transmission so figure around $3,000 for a truck with the Aisin transmission.
One last thing, when looking at used, particularly 3500 dualies you will want to know what transmission the truck has. It is easy to tell, open the hood and locate the transmission dipstick. The Aisin's dipsticks are on the driver side and the 68RFE's are on the passenger side. The Aisin tranny can pass more torque and has de-fueled shifts, it is a way to tell if the Cummins has the higher output calibrations (Aisin equipped). Depending on the model year, the option to buy the Aisin was around $2,600 but this option doesn't increase the used value of the truck, I would opt for the Aisin in used but I would never lay out $2,600 for new unless I had to pull 20K pounds or more.
One last piece of advice, these late model diesel pickups are holding their values. Before signing on a used truck, check the pricing on a new one at volume dealers like Dennis Dillon (
http://www.dennisdillonchryslerjeepdodge.com/all-inventory/ram-caldwell-id.htm), I've read where many folks bought for about the same price or a little more over a used one. Dillon offers a free shuttle service from the airport to their dealership. I almost went that route and at the last minute, a local dealer called and said he would price match Dillon.
