Welcome back. You're sure to get as many different opinions as posts, but I'll gladly give mine
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Basics... I prefer sugar sized aragonite sand for a substrate. Foster and Smith run good shippign deals (unless something has changed since I last ordered), so you don't get hammered with massive shipping costs. You won't need a lot for a 55, though. I usually go about 3-4" deep.
Leave plenty of holes and swimthroughs in the rock work.
I generally try to stick to 10X turnover or less through the sump. If you have an external filter with bio balls, you can easily replace the bio balls with smaller pieces of rock or rubble.
For softies and LPS, lower to medium flow is generally good enough. The larger outlet pumps (Tunze's, Vortech's, Koralia's, and even the Maxi jet mods) area good way to go because the flow is so spread out. I've just purchased one of the MJ mods and so far and happy, though I hear that over time they stop functioning properly. I run Tunze's in all of my displays and am a huge fan, but they aren't cheap.
Breaking the surface of the water is something I think is important. There are some surface skimming add ons to pumps/filters, but a drilled tank with a good overflow weir is the best way to go. Breaking the water and having it fall helps add oxygen and break up the film that develops (blocking gas exchange and light).
Mushrooms are some of the hardier corals, but IMO, as long as the tank is properly cycled and the parameters tended to, any soft coral and most LPS corals will fine with little attention. I run carbon in every tank regardless, but mixed tanks, esp. seem to benefit from carbon. I've never had any issues with coral warfare, but some have stories. Any (photosynthetic) soft coral should do well in a cycled tank, given decent quality lighting. Stay away from sponges, gorgonians (unless you know they are photosynthetic), Dendronephthya sp (Carnations, Chili's, etc.) and the like. They require special feedings and even with feedings, many still decline over time (still learning on those types of corals).
Lighting is as mixed a question as they come. I run MH, T5, and VHO on my various systems. I've got MH and VHO (250W DE's and actinics) over an SPS display, MH and VHO over a softie/LPS display (175W and actinics), T5's over a softie tank, and T5's over mixed tanks. T5's allow you to keep just about anything. They can be too bright for some corals (at least initially) since they have a continuous beam of light (halides spread from a point). There are usually not many dim spots under T5's (which can be a problem-rock ledges help for lower light corals). The softie tank with T5's is a 40B with a 4 bulb Tek lighting fixture. No problems with that. Some corals don't like the light, but for the most part, everything is happy. I've known tanks with just PC's or just VHO's that do great as softie tanks, too (though not a fan of PC's).
Get a top off unit. You can build them yourself for about $25-30. One of the best things you'll do for yourself (and your coral).
HTH
