and I second everything Sorseress said.
In some specific responses, it's almost always the wrong way to be thinking when people star asking "how small can I go" in the size of the octopus or the tank. It's easy to imagine that starting small is the best approach, but with octopuses, it really isn't. Octopuses of any size produce a lot of waste and are very sensitive to water quality, so a small tank, with a small water volume, can go bad very quickly and kill the octopus if anything goes wrong, while a larger tank changes more slowly and can help you notice problems in time to correct them and keep it safe for the octopus. Also, smaller octopuses are universally shy, nocturnal, short-lived, and usually less interactive... if you want an experience that shows a lot of personality, and will interact and be out in the daytime, it's much more rewarding to get a medium-sized or larger octopus like a bimac or briareus. Not that dwarf species aren't rewarding at all, but they're much less reliably so, and in some ways are harder to keep than their larger cousins. Larger octos like vulgaris and GPOs start to get too large to keep practically in home aquariums, so we recommend the midrange-sized octos as the most rewarding and easy to keep happy and healthy for a long time (although the heartbreak of keeping any octopus or cuttle is that they sadly are short-lived animals.)
The food cost is often high, since some octos will only take live crabs or shrimp as food, so usually the cost of feeding it is far higher than the cost of the animal and even the tank.
All octos can bite, but most rarely do. Some fairly smart people suggest being cautious, as a bite exposed to seawater can get nasty infections, but several octo-keepers here have been bitten without major health problems, and it seems to be rare. Blue-ringed octopuses should be avoided, though, since unlike most other species, their venom is very, very toxic to humans, and a bite can kill in minutes. They don't bite frequently, but we recommend avoiding them. It is not known if wunderpuses or mimic octopuses are venomous; as far as we know, no human has been bitten by these animals, but we don't recommend them as pets anyway.
I think all the rest of your questions (and maybe those, too) are answered in the articles and some of the major threads...
Usually, being patient for a few days will be rewarded by answers, I'd say waiting at least a day for a response is best before re-posting on the same thread unless you have something new to add. A lot of the most knowledgeable people only log in every day or two, and asking multiple times doesn't really speed that up...