If the cummins is a cm2350 instead of a 2250 they are better. Drive and shift like an older style engine and I know a couple guys with almost 400,000 on them with very little trouble. I have a 16 Pete with an isx but it's never been on the road yet so I can't really say how it's going to be. I had a 2012 that was deleted and it was very reliable. Had 15000 hours and 800,000 on it when I sold it. Never missed a day after the delete but I also had them keep the power down at stock 550.
Also if your looking at the t660 you might find a paccar motor. The early ones had a cam recall but have heard pretty good things about them otherwise.