My next setup will probably be about this size 55g/240l and I have been thinking extensively about the 'kit' I will be using. For a setup that can house pretty much anything you want:
For lighting: two 150/250W metal halide units ('builder or sign' unit with the bulbs changed to 150/250W 14000K aquarium bulbs) The wattage is really up to you just match it with the unit. 2x 45W T5 actinics. You can go with just the metal halides if you wish they are sufficient.
For water movement: Tunze wavebox (creates that brilliant natural alternating flow) alternatively you need several powerheads, the turnover depends on the corals you wish to keep.
For filter: A sump/ refugium unit probably about 60-100 liters with a return pump that does about an 8x per hour turnover (440gph should suffice) and 38W PC unit for the refugium. Live rock 30kg should be good for a tank that size most can be placed in the display with some broken down to live rubble for the sump. Skimmer there are lots of different ones available Tunze/Deltec/Prizm are fine just check what volume they can handle. If you go with a 'in tank' style skimmer that can go in the sump. A hang on skimmer can be placed on the edge of the display tank.
Test kits: Get decent kits; Salifert or JBL are good reliable kits. You need pH, dKh, ammonia, nitrite, nitrate, phosphate and calcium. Swing arm hydrometer is fine, just remember to rinse thoroughly after use. Some like to go that little bit further and use a refractometer. A simply thermometer is sufficient preferably with a suction cup to attach to the glass.
For heaters: go for 2x 150/200W heaters. One goes in the display tank and the other in the sump. Two is a safety measure less chance of two breaking down at the same time. (Although I am sure it will happen and does!!). Also keeps both volumes of water at the correct temperature.
Salt: Buy a decent salt and buy the biggest tub you can because it works out more cost effective. Seachem reef salt or reef crystals are two reliable salts that I always recommend. There are many others which may be good but these are the two which I know and trust.
Right now for explanations:
A sump is a separate tank (an old aquarium will do) usually of smaller volume, which is divided into chambers by partitions. Water from your display tank is syphoned in to the sump and then a pump returns it to the display tank after it has passed the filters. The partitions are either glass or plastic and the chambers are where various filtration can occur (depends on what you want to put in there commonly live rock/rubble). The refugium is a unit used to grow plants/ algae which act as a natural absorber of the nitrates and phospahtes. It can be incorporated into one chamber of the sump. This link:
<a href="http/www.youtube.com/watch?v=CxnugJGiT3o" target="_blank">http/www.youtube.com/watch?v=CxnugJGiT3o</a> will teach you exactly what you need to do to build a sump. This is part 1 of 2 but the link for part 2 is obvious when you see part 1. You can put macroalgae like caulerpa, chaetomorpha etc in the middle and the light source can just bridge across the sides of the sump. The sump serves a multitude of functions but the key ones are: to hide equipment and expand water volume thereby stabilise water chemistry. I would certainly invest in one if I were you, they can safe a lot of hassel in the future.
R/O water is reverse osmosis water. It is tap water that is purified by membranes to get pretty much pure water i.e. the minerals and ions are removed. There are two ways of obtaining it 1) get your own R/O unit. The more stages and more waste water it produces the better quality the R/O water (in theory). 6 stage R/O filter with 2 D/I units is a high quality bit of kit and will produce somewhere in the region of 98% waste water. 2) buy it from your LFS if they sell it. There are pros and cons to both methods so it is really your preference.
Hope this helps
Regards