From start to fish?
For starters, I need to know if you are going to drill this tank. This is common practice in Marine Aquaria, as a lot of us use a sump-style system. The 29 gallon would make a perfect sump for the 55 gallon. I suggest you read Understanding Sumps
. This will give you a detailed idea of the workings of a sump.
Working my up the tank from the floor: You obviously need a stand to store the sump uner (if you are going that route) and to hold the tank. Then you will need a Deep Sand Bed (DSB) for proper denitrification. Anaerobic bacteria that convert nitrates to Nitrogen gas (which leaves the system naturally) resides underneath 4" of sand, so we try to keep our DSB between 4" and 6". If you choose to not use a DSB, then use less than an inch of sand.
Next would be Live Rock. In a post earlier in this thread, Pasfur makes mention of Marco Rocks. Everyone on here that has used them has had nothing but good things to say about them. After filling the tank with sand, rock and saltwater, follow the advice of Pasfur earlier in this thread as to the maturing of a marine tank.
Lights! To properly answer this question, I would need to know the exacts of what you would like to stock. I'm speaking more towards the invert side of the spectrum. Are you planning on corals? What kind if you are?
Skimmer! I believe that Pasfur made some very good reccomendations for skimmers that would be suitable for a 29 gallon. The Aqua C Remora Pro should be able to skim a nicely stocked 55 gallon.
Let me know if you have any questions, or can answer any of my questions.