+1 on gas exchange.
Only other issue I see is making sure you're guaranteed a little one (6-8" max span) if you're buying on line. (Many places I've looked at won't guarantee size.)
Nitrates can be taken care of w/ water changes no problem. Detritus may be an issue as well. They don't need that much to eat, but they are messy eaters, and given their diet, CUC is a tough call, as they'll eat most of them.
Also be careful not to spook it when you're cleaning the tank. Mine inked me once. That's a mess...
It is fascinating to watch them hunt though - fun critters
P.S. I'm sure you're aware, but put as many colors and textures of rock in the tank as are marine safe. Octo can imitate just about anything on both counts.