Refering to some of the recent posts to this thread...
I would definitely look on ebay and in the classified adds for live rock and tanks, that is what I have done and I have gotten so much live rock, over 200 pounds of live rock in the past six months, and the total that I have spent is somewhere around $200, because it was all from people getting rid of their tanks. All my tanks are second hand as well.
I wouldn't cycle the tank with anything but live fish, but ONLY if I have live rock and live sand from a previously established tank first.
I personally wouldn't use a damsel to cycle a tank, they are very territorial and if you give them first dibs on a tank by adding them first, they will declare most of the tank their territory and beat up on other fish that are added later. Cycle the tank with one Clown fish is what I would do. But I repeat : have live rock and live sand in the tank FIRST, make sure that the live rock is from a mature tank and is already cured.
I have kept and octopus before, they are relatively spendy, and have a short life span, about 2 years. Keep in mind this is the lifespan of the small octopi that we would keep in our tanks, not the lifespan of the much larger octopi.
I would keep an octopus in a tank no smaller than about 30 gallons.
I also would think about using a sump in your set up, refugiums are great and on the "must have" list for many saltwater aquariests. I would also definitely get a protein skimmer, I use a AquaC Remora Pro, on my 37 gallon cube (it's overkill) and I have been very pleased with it's performance and will now only buy AquaC protein skimmers.(they don't use air stones)