Alright. Just finished up my English Essay. Now for the fun writing ;D
First and foremost I am no expert, but what advice I can give you is from my personal experience; reading a-lot of books, Forums, and first hand help from the guys at Jo-Mars (Ron).
I would do the biggest possible tank you could afford. Bigger is always better! (In more ways than one.. har har) That being said, bigger is always more expensive. However if your budget allows it will pay for itself in no time, IMO. The reasoning is because you will have very little fluctuation with water parameters and who says you can't have a blue tang in a 180 gallon? Avoiding the infamous "tang police" alone seals the deal for me ha ha
Sure there are plenty of other advantages to having a big tank... like awesomeness points, but I'm sure you're aware of that already. And I liked the idea of having a 55 gallon sump! That is such a great start! Imagine all the cheato you can harvest in there for nutrient export (to help avoid nuisance algae).
Now when you take the plunge the costs are going to be extraordinary. Because you have to buy lighting, substrate, live rock, power heads, filtration, and salt. As said later on Metal Halide is the way to go! But t5 lighting is pretty rad too.
Substrate is easy. I have had great luck with the "supposedly live sand" you see in pet stores. Regardless if it's live or not it's appealing to the eye and just the right size, IMO. I'd go for 2-3 inches deep, but there's plenty of debate on that and I'm sure some more of our seasoned guys will chime in for this.
Live rock is where it gets you. I believe the general rule of thumb is 1-3 pounds of live rock per gallon. It is a necessity for reef tanks. There is so much science behind it, long boring story short a healthy reef needs live rock. This is where it gets fun and not so fun. The good news is you don't have to add all of your live rock in at the same time (although it's probably better). The bad news is it's IMO the most expensive part. I'd look online for real live rock. Go for uncured IMO. That'll cycle your tank and add great diversity. The curing process is not hard just long and sometimes smelly.. Some good live rock vendors are Premiumaquatics.com (Bali Alor) or Tampabaysaltwater (Caribbean aquacultured). Both types of rocks have there pros and cons.
Another thing to a successful reef is water motion and LOTS of it. Be sure to stock up on power heads and position them appropriately to eliminate as much of the "dead spots" as you can. A good rule of thumb is to have your aquarium's displacement turned over 20-30 times per hour. More never hurts IMO.
Filtration is a big one. For a tank that size you definitely need a Protein skimmer, sump (your 55 gallon), heater (can go in sump), refractometer (to measure salinity), and an RO/DI filter. I'm not personally "suave" with DIY sumps, but I'm sure they're lots of great write ups in our DIY section. I would avoid the hang on back filters and canister filters. They are unsightly and IMO just not necessary. Your other primary filter is live rock/sand. Not sure what type of filtration you already have, but you could probably salvage a-lot of your freshwater stuff, if you were on a budget.
And lastly SALT you need a-lot of it when first setting up a tank. However once all your salt is mixed and your tank is cycled, salt isn't that much of an expense. I change my water every 2 weeks or so. I think you change 10% of your water during a water change. So essentially what you take out, you put back in with freshly mixed salt.
I know this is all a summary, but I hope it gives you somewhat of an idea of what it takes to start up. Once you got those requirements satisfied, you are well on your way to becoming a reefer/tang owner If you do take the plunge, you won't be sorry ;D
I have built both stands and inwall tanks. I think the inwalls look better if done correctly and are usually less costly as you can skip the price of buying high quality woods you would need to make a good furniture grade stand.