With spokes lasting over 300,000 miles reusing them is a great way to save time (no need to remove old ones, re-lace, correct spoke lines, and stress relieve) and money (potentially $3 each for bladed spokes); although as a shop I wouldn't accept the warranty and reputation risk that a different builder had properly stress relieved. This also requires replacement rims matching the originals' ERDs.

Alloy nipples are reusable too provided you had the sense to properly lubricate the threads. Zinc anti-seize (aluminum is more common) is ideal because it corrodes preferentially to aluminum, although grease which keeps water out is often OK. Do that and you won't have problems with damage at 130kgf using a well-fitting two sided spoke wrench. Many people don't so their nipples freeze solid.

I used the same DT 2.0/1.5mm Revolution spokes except 2.0/1.8mm Competition rear drive side and alloy nipples in the wheel set I usually rode from 1998 through 2012 through several rim replacements with the last in 2010 or 2011.

I never broke any spokes and the nipples still turn as well as they did when I originally built the wheels in spite of riding next to salt water for years.

Wheel building is no more difficult than adjusting a front derailleur, although it takes longer with a lot more points of adjustment. Unfortunately doing it both well and fast enough to produce hourly profit like cable replacements takes experience mechanics don't get with the move to boutique wheels, with most shops compromising with poorly but profitably. Consequently consistently good results require doing it yourself or delegating to a one-person shop where the hands that earned the reputation are the ones which build your wheels.