it's not only the miles, it's also the age. That pump is 13 years old...
I hate to break down on the side of the road. So I rather spend the money on the parts and install them myself (if I feel confident that I can do it) than paying a tow truck or pushing the car over a busy intersection.
However, if you don't mind the hassle, then you start with the cheapest and work your way up. (Again, with my kind of luck, it would be the last part that I replace that will fix the problem, so I replace them together.)