Actually, 36 stainless spokes are a significant cost (at least to me). New spokes might be better, because old ones may have flat spots where they've fretted on the hub flanges or against each other - not on a good, hand-built wheel though (shouldn't be any fretting).
But if you're going to the trouble of re-building a wheel, and you're not certain the old and new rims and hubs have identical dimensions, you might as well get new spokes anyway - and get them in the right length. Bear in mind, too, that on a derailleur rear wheel (or any wheel that's not symmetrical), the two sets of spokes (left side and right side) may also be different lengths. And different tensions. And building an asymmetric wheel isn't entirely straightforward.
In fact, if you want to have a go at building your own wheels, by all means do so - it's not THAT hard. But if you're going to change rims and spokes, why not change hubs too (it's not going to up the cost very much more) and then you'll end up with two pairs of wheels, and you'll be able to continue using your bike while the new wheels are in the works, rather than rushing the build because you need the bike.
If it's a commuter or utility bike, have you thought about using a dynamo hub? There are some really, really good ones about now, and now's the time to think about it.
