From my research, it sounds like this UniFi system is pretty much exactly what I need. It can do PoE either through included adapters or a new switch, it handles wireless roaming, and if necessary I can connect to APs wirelessly. I don't know if it uses WDS for this, but it sounds like I can either connect two units on the 5GHz channel (if I get APs with 5GHz), and use the 2.4GHz channel for the clients. Pretty nifty...

I see that it also requires either an on-site PC running their software, or some sort of hosted account. I think the easiest would be the on-site PC, which would certainly cost less than most controllers I'm seeing.