for sure you will need different spoke lengths, at the very least on the disc side.
remember it all depends on how the original wheel was laced ( radial, 2 cross, 3 cross etc) and how you want to build the new one (radial on non disc, 3 cross disc side and so on.)
Im lucky that i have a mate that builds my wheels for nowt but he can build one in 30 mins so long as its a good quality strong rim.
Dont know why people are complaining about £1.50 a spoke, the ones in my roadie are £4.50 each.
Andy