As far as mileage on an emissions equipped diesel, I would be more leary of a truck with low miles versus one with higher miles. Lower miles generally means a lot of city driving and idling which causes a lot of emissions equipment issues. Higher mileage generally means a lot of highways miles which is the perfect condition for emission equipment. About 15-20k a year would likely mean that it was mostly highway driven. If there is a way to check the idle time, I would prefer to stay under 10-15% idle time compared to total hours.
As far as that specific truck is concerned. There were a lot of fuel pump failures in those years causing $10k worth of damage. It is a hit or miss depending on how the previous owner took care of it along with other factors. It is not a case of if, but when due to the design of the pump.