With that boat you better not worry about the price of fuel.
Not to rain on your parade, but hasn't the 6.0 liter diesel been a reliability nightmare? Are those teething problems cured in 06? I have found that the earlier 7.3 trucks are often more $ than the later ones for this exact reason. It is like 993 vs. 996 ...
That all said, 15k is a bargain for a 2 year old truck. That's a $40k+ machine.
George