Prepurchase questions, things to look for?

Currently have a 2008 ram 6.7 with a g56, and realized after installing double disc clutch that I want an auto or something else. I've decided to keep my eyes open for a nice used 2012-2014 ram... and was looking for stuff to look out for.. .and a few questions

1) Is the 68rfe a decent transmission? I had 47re or whatever in an old cummins truck I had, and it turned into quite the nightmare and quick (with totally stock truck). I dont intend on tuning the motor or anything, just leaving bone stock.

2) Are there any common problem areas you guys can recommend I look at before purchasing? I've got terrible terrible luck buying used. (my current truck has clean carfax, but found out they didnt change fuel filter in like 30k mi, cab is diff white than the bed, bed was kind of beatup under the bed liner etc) I'll know to look out for those things.

3) DEF, was 2013 the first years of the DEF? And does DEF improve mpg vs just the old regen where it dumped diesel into the dpf?