I can't really help with hang-ons,,, I have a CPR BakPak on my 37 reef which seems to do a good job,,,, dunno how it would fare on a well-stocked 55g though. I hear good things about the Aqua C Remora skimmer, I think they're designed for tanks up to 75g,,,,,but I've never used one myself. I'd stay away from the Prizms, VisiJets and Seaclones,, they're cheap but they don't seem to have much of a fan-club.
If you add a sump eventually, I can't recommend the Euroreef Skimmers enough,,, I have an ES5-2 on my 75 and an ES5-3 on my 90 and they are just perfect,,, I can't fault the design or performance.
If you plan on maybe adding a sump later, then try getting a BAkpak or Remora to get your feet wet (not literally),,, I got my used BakPak for $60 on Ebay,, might be a good place to start looking once you decide on a model.
Good luck!