As someone who has been in the middle of developing motor controls (also DC switchers -- lotsa MOSFETs for the 200A@28VDC we were dealing with as our largest design), I say just buy a friggin' power supply.
There's a reason the higher efficiency ones cost more. Everything with good efficiency probably has PF correction, and there are some sexy little chips that can basically take care of it for you (adding more switching, of course -- yeah!). Basically, it's easy to design a power supply. It's even easy to design one with pretty solid outputs. Will it be cheaper? Maybe, but probably not, and it's unlikely that you'll end up with a design that's efficient and if you want to you're going to put in a lot of effort to do what others have already done at pretty low prices.