In addition to Trent's answer I would suggest ordering one with disc installed at the factory. The disc brakes are hydraulic, so all wiring and plumbing are done in house. By plumbing I mean the hydraulic lines running from the pump to each wheel.
I'm a believer in 'if the factory offers it and you want it then order it.'
I guess it'd be like ordering a new truck and when asked if I want the electric retracting side steps and I say 'I want those, but no thanks. I'll get them installed later.'
I actually have those steps and they were installed at the factory. And it was cheaper than buying a quality aftermarket set and them installing them myself like I did on our previous truck.
Also, your new disc brakes now fall under the factory warranty.