20 amp breaker and 12 gauge wire, minimum. The wires can be run inside the walls as well as the drain, but that will take some doing-rmvl of interior walls is not an easy task. But, the wires could be run along the inside in a panduit (brand name) type of raceway, or wire mold or? I do not know where the breaker box is on your unit, but a conduit type of raceway along the very top of the inside ceiling will not be visually perfect, but certainly acceptable, as well as keeping it easy to run, and out of your head room-plus, no monkeying around routing through cabinets, etc. you need to visit a local electrical supply house to see what products are available-if they will not sell to you direct then, and only then contact an electrician. The big box stores may have something that will fill the bill, but I doubt it. Now on the drain, my pea brain says that water you keep outside will not cause problems inside. An outside drain may appear hideous, but it will keep the condensate outside. besides, if you're gonna mess with the smooth roof by putting the A/C up there, then a clear condensate line over the edge shouldn't be a big deal, either. That way, maintenance is a snap. I have overlooked the obvious, how are you mounting the A/C? I do not know what your roof support looks like. If a major remodel is in order just to mount the A/C then, runnning stuff in the walls would be best.