I would consider looking at pre built bikes and just adding a rack to that. building up a bike is pretty expensive.

The first frame you chose is a road (touring) frame, it drops vertically for a derailleur. For one speed, a horizontal dropout is more ideal so you don't have to have a chain tensioner or actually take links out of your chain anytime you changed your ratio.

maybe check out the Motobecane Fantom Cross or something