Buying a new (to me) diesel, need advice.
 

I have owned lots of trucks and lots of trailers, but I have never owned a diesel and have never owned a 5th wheel. Sometime in the next year or so I will be buying a 40'+ trailer in the 16K to 18K pound range. Obviously it is going to be a diesel dually, but beyond that I am pretty open to suggestions. I don't expect to be towing every day, but fuel mileage is a concern. I would like to say that money is no object, but money is ALWAYS an issue, so I am definitely going to be in the used market. However, I am not opposed to spending big dollars, if it is the right thing to do. Primarily I am concerned with performance, specifically, torque. I know the newer (DEF) models kick out quite a bit more than previous motors, but I have also heard there may not be as much room for improvement with aftermarket products. So, in a nutshell, my question is this: Am I better off buying a mid to late 2000s model and improving performance with aftermarket stuff, or just buying a 2011+ model that already gets 800lbs? Obviously, I would rather spend $20,000 rather than $40,000, but when it really comes down to it, I want the best truck for the job. Let me know what you think. Thanks for the help.