Protein Skimmer - $200-400 (anything cheap (asm or similar) may need to be slightly modified, if you want something to work, $500+ potentially.)
Pump/Power head - $30-60 each, will want at least two, TUNZE ($60 for the 6025's), or Koralia's (#3's or #4's)
Filter - Dont need this protein skimmer is closest thing to a filter you will have.
Heater - Two, Jagers are nice, I also just use hagens, at least two.
Live rock - 1.5-2 lbs per gallon (75-150 lbs for a 55g) marcorocks.com would be cheapest route, but it is not live rock, so you would need to take things slower, local stores will sell it to you for 6-12$ a pound. can buy online for 2.50-5 # a lb shipped to your door.
sand - just get dry stuff in a bag, whatever the pet store has, do NOT buy bags of "live" sand, if you REALLY want to spend a bit more money buy ONE bag only, no need to buy ALL live sand, no need to buy any live sand really.
Hydrometer - yes $45 from dr f&s do NOT get a swing arm
thermometer - just get a mercury that you can check the temp whenever, no need to alwyas be able to see the temp.
Lighting - totally depends on your tank if you want corals you will need to go with HO t5's and/or metal halides. for a 55g two 250w metal halides or 4 to 6 x 54w t5's would allow you to house anything and everything. if you are handy you can save a good bit of money by buying retrofit lights to put in a canopy or you can buy complete hoods. if you are not having corals, then the cheapest light that makes you fish look good to you. cost est $300-1200.
I would buy the following:
(2) Tunze Turbelle NanoStream Pump 6025 ($140 (i think you can find them about 60 each tho)
(1) ASM G-2 Protein Skimmer w/ Sedra 3500 Pump $290 (i would modify the needle wheel pump and turn it into a mesh wheel, these skimmers work pretty well then. (you could also get away with the g-1x for a few less $$$)
(1) Marine Depot Refractometer $50 ( i am looking on marine depot for prices right now.... can buy the same one from anywhere, dr F&s sells them for $40 sometimes)
(1) merc thermom $2 ... from wherever (petsupplies plus)
(1-2) Red Sea Reef Base 44lb. Bulk Box prolly want about 80lbs of sand these at $100 each, a bit steep you can do better on the price locally, carib sea or whatever agra doesnt matter.
(2) Hagen Aqua Clear Compact Preset Heater, 200 Watt $26 each, i have maybe 3 of these in my tanks, a stealth and a jager, doesnt matter to me, never had problems with heaters, i just have a lot of them to keep my temps up at night (also have 250 ish gallons of water to heat)
(1) Live Rock -
http://www.reefhotspot.com/store/index4.php?cPath=60 $100 for 44 lbs not bad, plus whatever shipping.
(1) marco rocks base rock -
http://www.marcorocks.com/index.asp?PageAction=VIEWPROD&ProdID=40 this is 50lb box for $130.. i know you are like, wait thats more expensive for dead rock... but no this is DRY rock, nothing in it so it is really light. you get a lot of rock.
(2) 250w Lumenarc Stealth (HQI) Metal Halide Retrofit Kit from hellolights.com $578 i would select the pheonix 14k bulbs. also this would be in my own custom cuilt hood so i would buy...
(1) 48" 2 x 54w T5 HO Retrofit Kit with one actinic r and one reef blue bulb $148
You will also need a sump/refugium to put most of this ugly equipment (skimmer, heaters, extera rock, more sand etc.
I would buy a nice sized tank that fits in yoru stand and turn it into a sump/refugium combo (melevsreef.com) either a 20 or 33g long aquarium should fit.
then you will need a return pump.. a quietone series pump would do.
if your 55g is not drilled you will need an overflow box or drill the tank, drilling 55's is a no-no d/t most being tempered glass which means BOOM shatter. so lifereef.com pricey... but not something you want to skrew around with.
I will look over this after posting and see what else you need to buy.
Oh yea, airwaterice.com typhoon III ro/di unit $200 appx. you will want one of these. i love mine, you can buy whichever you want, there are many out there. i constantly get 0-1 TDS from my typhoon tho, not sure everyone can say that about their ro/di unit.