the mad norsky hit it on the head. No matter what you buy you can be afraid it will break down at any time. I have a ford 6.0, paid $4000 to have it 'bullet proofed', now dont worry about it at all. It runs great. I have had friends with chebby and dodges and they have had their share of problems.
If you decide to buy used pre-DEF then do the research for the vehicle you like. If you buy new well its a******shoot. Last summer we passed a new dodge struggling to crest Wolf Creek pass and when we later saw him it was again by the side of the road! So they all can have issues.
Reading all the doom and gloom on the different boards is what causes anxiety, get a vehicle have it checked by someone who knows the vehicle line and go from there. If willing and upgrades have not been done, get em done. The new fords and chebby's have supposed fuel delivery problems, they may be fixed by now dont know. I have heard of some issues with the new dodges/ram but I dont really follow them.
Whatever ya do, research and service as required/desired.