I'd probably look into a CSL Power Compact hood with moon lights. My price on that is about (without looking it all up) $250.00 and you might beat that by shopping around. That will give you 4x65w PC
lamps, moonlights, and fans all installed in a hood that doesn't look bad at all. You'd be able to keep about any of the low/medium light inverts with that setup on a 55 although you might have to be careful about placement...higher light inverts close to the top.
If you are willing to buy/build a canopy for the tank, you could save some $$ by going with a retro kit of some sort. I think you could come in just under the $400 mark with a pair of 175w MH
's and a 2x65w PC
actinic by doing it that way. Or, if you wish, you could go with a PC
retro kit in a canopy. I'm thinking the best deal for you would be the CSL PC
setup given what you want to keep and what you want to spend. If you think you'd ever want to start keeping some of the higher light inverts though, you'll be better served to go ahead and invest in a MH
system. Having been there, I can tell you that it stinks to spend a ton of cash on a lighting system, decide you want a clam or some SPS
corals, and find out you're going to have to drop more $$ to upgrade the lights even more.