About food:
I've only had one octopus, but I've had it for almost a year with no problems, and it's a bimac, like you want, so maybe my experience is applicable. I have a wild caught bimac, and she eats pieces of thawed frozen shrimp and scallop from Costco, so food costs less than a dollar per week. I thaw a little piece of shrimp or scallop in a cup of tank water, pierce it loosely on a long skewer, and then wiggle it around in front of the octopus. The octo likes to chase it a little and wrestle with the stick (and me) for a minute, all of which makes it feel like it stalked, subdued, and killed its dinner, so I play along. I feed a cube of meat where the distance along one edge of the cube is about 75% of the distance between the octopus's eyes, and I give that much every two or three days. My tank is cold (63 degrees) so my octo digests slower and eats less per week. Feeding is really easy, and really cheap. When I go to the beach at low tide I grab a shore crab or a muscle as a treat, but the shrimp and scallop make up 95% of my octo's diet. Be sure that the shrimp you buy are marine shrimp, not freshwater shrimp, and that they are raw, not cooked.