Please do some more reading. I spent about $800 to set up a 29 gallon tank so I could get an octo about a week before I discovered this site. You could probably keep one of the dwarf species in your (or my) tank, but any of the "normal" species would require something much larger, on the order of like 60 or 90 gallons (I'm sure one of the more senior members will clarify). I've now given up hope on keeping an octo in my 29, I'm just enjoying learning how to keep a reef at this point. It's coming along well for me, it's cycled for 6 months and I've finally started adding some corals and inverts. I did have an octo in my 29 at one point, and I'm fairly confident that had I not purchased a fully mature specimen that was on the edge on senilence, I probably would have killed it anyway from lack of experience. It's also worth pointing out that my octo was an Abdopus aculeatus, one of the hardier, more common and easier to keep species. A mimic or wonderpus are both much more sensitive and harder to keep. I would recommend doing some reading on the subject and learning. I can't really advise because I'm still learing myself, but I can assure you that from reading your questions, your no where near ready to keep one.
Please do some reading and educate yourself, these critters are very specific in what they need and are NOT for beginners.
This site is a great wealth of free knowledge, use it. :)
It is great you are interested in the hobby of keeping a ceph but you should definitely do your research first. Cephalopods are the most difficult invertebrates to keep you should try keeping a reef tank if you have not already and gain some experience. The tanks you spoke of could possibly hold a dwarf species but not very comfortably and lastly I would suggest looking into a hardier breed as the Wonderpus while gorgeous is extremely difficult to keep due to they normally live in a muddy environment which is difficult to keep stable in the aquaria. There is a great book written by Colin Dunlop and Nancy King who I'm pretty sure are both on here called "Octopuses and cuttlefish for the home Aquarium" that could answer a lot of your questions I highly recommend giving it a peruse or just scan through the forums you'll find tons of very useful information some even directly from Marine biologist.