I would say get new spokes and get a proper nice tensioned wheelbuild. Your old spokes are probably kinda worn out and stretched by now. Id say go with Sapim spokes or maybe DT spokes, those are nice. By using new spokes I think your wheel will just be that little bit stronger, which is alwaysgood and most likely worth the exrta money. OF course I have no experience with using old spokes so if someone with experience pipes up and says its totaly fine then go with what they say. O yeah, why are you lacing a Moment hub to your Alex DX rim? I though you were buying the whole KH '07?
You should get new spokes since they aren’t as strong when you reuse them. The easiest way would be to just get them from Darren or UDC and just tell them you need the spokes to fit that rim and hub and that way you won’t have to figure out what length you need, they will figure it out.
100%…go with new spokes! Different hubs stress spokes in different ways. If you use old spokes those spokes will have been stressed in different ways by two different hubs and will most likely break or have a wheel that will not stay in true. You might get lucky, but I wouldn’t risk it. Even if you were using the same hub and rim I’d suggest new spokes. Building it up yourself? It’s fun and frustrating at the same time, but gives you a nice feeling of accomplishment…good luck!