Reusing old spokes is fine so long as the length is the same and they’re not damaged, e.g. chain going over the back of the cassette and gouging them.
(Hmm, is there an echo in here…?) 😉
The only thing new I would add is that I’ve reused many old spokes but do rather like to use new nipples as older ones tend to be a bit corroded/cracked/rounded, especially aluminium alloy ones. They’re cheap as chips, just get the correct diameter, measured near the start of the thread (usually 2.0mm, though über-lightweight ones might be 1.8mm). I may be set in my ways, but tend to avoid alloy nipples unless someone requests them – they’re not a great idea for long-term wheels.
Good luck, and remember the advice that has got me through building hundreds of wheels: “It’s not rocket science!”