Tommy, your question is one of the top questions that are floating around in the interweb. In my opinion, since I have been doing this full time, DEF is one of the worst things that happened to owner operators with not so deep pockets. Since it’s inception in 2010, we as a company have seen millions of dollars go out when it comes to exhaust repairs. If I were to buy a tractor, I would probably look into a glider kit the most. If your not sure what a glider is, they are brand new trucks, less engine and transmission, and you put your choice of both in. You get a 2017 tractor, but with no emissions because the USDOT recognizes emission year from the engine, not the tractor.
As far as manufacturer, between Freightliner, Volvo, International, Peterbilt, etc. I would say they all have their pros and cons. Most of the recent decisions made by owner operators is creature comforts and recently driven decisions. I can get into repair costs by manufacturer if you’d like.
If you have any other questions, please feel free to post them and I will do my best to answer, or do some digging to find out.