Burning_Ranger opined the following...
I've ordered a bike without disc brakes (forks are disc compatible), gonna
come tomorrow. What I want to know is it possible to use disc brakes by
simply replacing out the old hub with a disc hub? I realise the wheels will
have to be re-built, will a LBS do this for me and how much is it likely to
cost?

It'd probably be worth having your LBS "face" the disc mounts (Grind
them so they are perfectly perpendicular to the hub axis) before fitting
discs if none have been fitted before. If they are fitting the discs
they probably ought to do it by default.

Other than that, it's just a case of new wheels. Costs seem to vary
dramatically depending on your LBS. If you have a choice, it's probably
worth asking around to see who builds good wheels locally (Or learn.
That way it's your fault!).

Jon