Welcome! First, start reading around the forum here. We've got plenty of content that will help you make a decision. Overall, I would make sure you have some kind of extended warranty.
Welcome! Do you get a lot of highway time, rather than city driving to allow the truck to perform a proper regen? Does it ever warn you, exhaust 80% FULL, etc?
Welcome! A pretty common problem with the ecodiesel. I would recommend searching "throttle control" or "ETC", sample: https://www.ecodieselram.com/forum/threads/ecodiesel-service-electronic-throttle-control-message.203/
Welcome to the forum.. Check out this section for more information on potential tuning/vendors.
https://www.ecodieselram.com/forum/forums/ecodiesel-performance.28/