
to TONMO!
It sounds like you've already done this, but just in case, make sure to read the ceph care articles in the tab at the top of the page, these answer a lot of common questions.
That sounds like a great size tank for most of the species commonly kept. I'm sure some of the experienced tank-keepers will chime in with more details, but it sounds like you're on the right track. We recommend getting a tank-bred bimac if they're available, Zyan Silver has been breeding some, and he may have some available now or in the future. Tank bred octos are usually healthier, happier, and they're a known species and age so they're much less likely than wild-caught ones to be close to the ends of their lives.
Octopus briareus is another species that does fairly well, but they're not available tank bred.
TONMO recommends strongly against keeping blue rings. They're really very, very venomous, and their bite can be lethal in minutes. Often fish stores will downplay how dangerous these animals are. Remember, octopuses can even escape their tanks, so you could step on a blue-ring unexpectedly on the floor or something, and it is would be far more likely to kill you than a rattlesnake. If you were to have a blue-ring, unless you're suicidal, you would be able to interact with it far less than other octopuses you could choose, and you'd have to take all sorts of precautions if you had to clean the tank, move the animal, repair filtration, take care of "octo-proofing," and so forth. By most accounts, they aren't as personable as some other species, either, so you get "less octopus" and have to deal with a bunch of extra stuff. Roy Caldwell, who's kept blue rings in the lab for research, has written an extensive article about them
here if you are considering buying one from your LFS, but pretty much everyone agrees that they don't have any benefits that outweigh the liabilities.