Used 2011 F350 6.7. Do I need to purchase warranty?

I would not want to own one without coverage. That said... most aftermarket "warranties" are not a true warranty, and contain enough exclusions to make them worthless.

I'd suggest sticking only with Ford Extended Care (if it's even eligible) or Mechanical Breakdown Insurance through your insurance carriers. And to that point, stick with your carrier (or change to carrier providing the MBI) that way there's less chance of finger pointing in case of a HPFP failure.