I'd say you should do some more research before buying a 2003 - 2007 Ford (6.0L) or 2008 - 2010 Ford (6.4L) diesel. They were both made under contract by Navistar International, very, very troublesome by most accounts. Ford even sued Navistar over the cost of warranty claims for these engines.
The 2011 Ford diesel was a new, in house (Ford) designed 6.7L diesel which was much better, but then it's the very first iteration of a new engine which most will say to stay away from. But a huge improvement to be sure, and likely every bit as reliable as a 2011 Duramax or Cummins.
On the truck chassis, 14,000+ is going to likely exceed a few of the ratings of an F250, or any other 2500 series truck, and will almost certainly require beefing up the rear suspension (timbrens, airbags, extra leafs, etc.).
I think you're in 350/3500 territory with that 5er.
Have fun shopping.