More than likely the actual size is 72x18x24 which comes out to about 135 gallons (I had that sized tank about 6 years ago...nice tank). That is a fairly common size that many manufacturers make.
To use the 55 as a sump, you could try not using any baffles but most do prefer to put in some baffles. You could either get some glass pieces cut and silicone them in (that would be my preference) or try to glue in acrylic pieces. The problem with using acylic is silicone won't properly bond to it, so they can come loose fairly easily. As far as where to add baffles, my best suggestions would be to google a search for aquarium or reef sumps and see what others have done and copy that.
For a skimmer, I would recommend looking at the AquaMaxx line of skimmers. They have some really nice insump skimmers that would work great and the prices on them is very affordable. There are tons of other skimmers out there, so just take a look around and feel free to ask about any of them.
For lighting there really are three popular types of lights to go with: Metal Halide (with or without supplemental lighting), T5 and LED. Halides are nice and most are fairly affordable, but they can cause extra heat to the tank so you MAY need a chiller to help during the warm months.
T5 are good, but it would require quite a few bulbs over your tank. Overall the tend to run cooler than halides and many systems are fairly affordable. The type of corals you want to keep would dictate the amount of bulbs you would need to get (more light demanding corals will require more bulbs, less light demanding corals less bulbs). More than likely you would need a fixture with at least 8 36" bulbs.
LED's are very popular now, but unfortunately also tend to be the most expensive initial cost wise. But there are no bulbs to replace for a long time (years) and they generate very little heat and are cheaper to run on your electric bill.
Have a look at this article on Items needed for a reef tank. I think that will cover most of the stuff you need to get for a reef tank...but please ask away if there are questions on specific items.
"Simply put, you believe that things or people make you unhappy, but this is not accurate. You make yourself unhappy." Wayne Dyer