Wheelbuilding is not so difficult, it requires patience.
If the old spokes come of easy, reuse them, if they don't, don't.
I keep some old spokes to replace broken ones. If you use new ones you have to retighten them later, old ones are 'pre-stretched'.
So: don't mix 'm up, use new OR old spokes in one wheel.
Rims: I buy them used, because the black 28"x1-1/2" rims I prefer to use are all powdercoated these days, and start to rust under the coating. You can't powdercoat things for outdoors use without a protective zinc layer or something like that underneath.
As long as you don't use the rims for braking, a wee bit of woble is not so much of a problem, less then two millimeters is hard to notice when riding.
This is what I use:
Go ahead, try it, it's fun to do, and even more fun to ride on a home made wheel. It does not have to be perfect the first time, you have to start somewhere.