Welcome to the forum! I would advise to drive it and see how you like it. The EcoDiesel is great on fuel mileage and if you don't tow very heavy, it's a great truck.
"Stacking" tuners really isn't needed anymore and is old school.. GDE (or any custom tune for that matter) can get just as much out of the truck as two devices put together.
There's a transmission harness with a connector up beside the trans itself, check it and be sure it's not loose. I've heard of many weird issues if it's wet or doesn't have a good connection.
Check out Amazon or Rock Auto:
https://www.rockauto.com/en/catalog/ram,2016,1500,3.0l+v6+diesel+turbocharged,3353660,engine,oil+filter,5340
Amazon: https://www.amazon.com/dp/B00KQ4VE64/?tag=jhdiesel-20