welcome and thanks for joining TONMO! I'm sure one of our resident ceph-keepers will be able to help, but 55 gallons traditionally "in the realm" which is good... it does depend on species (bimac would work).
Welcome! I have a 55 gallon bow front tank that I’ve been keeping octopuses in for 12 years. I’ve had lots of success with O. briareus (Caribbean Reef) and A.aculeatus, among others. I’ve never kept a bimac as I don’t have a chiller.
One of the biggest problems when it comes to obtaining an octopus is that frequently the seller can’t actually ID them for you. On-line sellers based here in the US that are collecting octopuses here are generally pretty good at giving you an ID. I’ve found that while lots of the LFS around me can order them, they are usually TOTALLY CLUELESS when it comes to what species they’ll get- to the point where they don’t even know what to ocean it’s from! Frequently you can get aculeatus from Live Aquaria, but again it’s a bit of a crap shoot.
The other tricky part of this game is that octopuses are seasonal. If you got one now, it would most likely be close to senescence, which means you won’t have it long. I’d start looking for one in early spring!