Spokes can be re-used if they're in good shape. Nipples can be re-used too, if they're brass and aren't stripped. If the nipples are aluminum, just get new brass ones.

But spokes on a front wheel are always 3-6mm longer than the spokes used on a rear wheel with a rim of the same ERD. You won't be able to re-use them in the back.

Double butted spokes is the best way to go, if they're 15/14 gauge (2/1.8/2mm).