I've only kept bimacs, but from the reading I've done, I've always thought that for people that want a bimac, but don't want to deal with a chiller, an O. Hummelincki would make sense. They are slightly smaller than a bimac, warm water, and are reported to be diurnal and have great personalities. I understand they can also be purchased, at least at some times of year.
Rocks and sand:
If you go with a bimac, and chill the tank, I think you can use tropical "live rock" and sand. Tropical live-rock provides bio-filtration, although it will work at a slower rate than it would in warmer water. For that reason, you should use more of it (1.5 lbs per gallon?) and feed the tank less. I didn't think tropical live rock looked right in a Southern California Biotope tank, and I'm too cheap to pay for live rock, so I used native (non-porous) rock, which provides no bio-filtration. I use a wet/dry trickle filter, skimmer, and deep sand bed for bio-filtration. If you want to keep a lot of other cold water animals, most of which need to be fed a lot because they don't use light to make food like tropical animals do, then even 1.5 lbs of tropical life-rock per gallon is probably not enough bio-filtration to keep up with the mess made by all that feeding, so you would probably need to supplement the live rock with something else (like bio-balls) and have mechanical filtration, and an overlarge skimmer. I should also mention that the two bimacs that I've sent to Tonmo people, and were kept (I think) at room temp, with tropical live-rock, didn't do very well, and died early of (I think) unknown causes. I'm not drawing any conclusions, but one possible reason is that they caught some exotic disease or parasite from exposure to the tropical (exotic) live-rock and/or live sand. I'm not even saying that that's a likely cause, it's just one possibility to consider when thinking about keeping a bimac.
I like catching my own bimacs, and other tank inhabitants, but if I just wanted an octopus of any species, I'd try to get a Hummelincki and avoid the headache of a cold water tank, while retaining most, if not all, of the advantages of a bimac (personality, diurnal, moderate size)