If you don't mind drilling (or having the tank drilled) and adding some kind of overflow box, a sump is by far the better way to go for filtration. If you have the tank drilled, lower the hole about a half an inch below the standard drill positioning to allow air space at the top. This little extra tank space without water is helpful in keeping the octopuses inside the tank. If this was not for an octo tank, I would suggest using a sump when you can for both asthetics and ease of maintenance. Even with a small sump you add just a little more water and octo proofing becomes much simpler since your skimmer will be in the sump. There are a variety of sump options but I use a very, very simple set up with mine. In stead of an official sump box configuration, I use a standard aquarium tank, run the tank water to a 7" (called both 7" and 8" but they are the same) diameter filter sock and place a bag of charcoal in the sock. I rinse my charcoal weekly (I actually keep two bags and swap them each week, rinsing the dirty one and putting it in clean RO water for a week). This kind of open sump gives a nice surface area for a cooling fan if you need it and a safe place for a heater in the cooler months.
One thing you don't mention is your water source. You will need RO/DI or distilled water for both your salt mix and your freshwater top offs.
You need live rock (30-50 pounds) and I recommend at least one cluster of giant purple barnacle shells for a merc tank (to increase the likelyhood one will choose it for a viewable den).
You will want a red light for night viewing. No other light is needed for a nocturnal tank so if you can replace the current lights with a red bulb, that will work. Unfortunately, if the set up you are looking at has a light fixture, it is likely a fluorescent tube and I am unaware of any in red.