1. Another poster mentioned you may not have to cut the shift or brake cables. That's contrary to my experience. An 18 year old bike is almost sure to have frayed or kinked cables that won't slide through the housing or anchors. If it was my bike I'd cut the cables but keep the housings intact so that you can use them as templates for sizing your replacement housings.
2. Repainting a bike is a fairly labor intensive project. If you're going to do that much work, you'll probably want it to operate smoothly when you finish. New cables and housings will work wonders in that regard.