Getting a certain species is difficult, you'll likely end up with a "common" octopus but most are not that hard to keep, despite what is commonly said. Most have short lifespans, but not just in captivity, most dwarf species only live less than a year in the wild so most will live for about 6-12 months in captivity.
I kept one before and it was cool but like already said somewhat overrated. They are nocturnal so don't count on seeing it with the lights on, I never saw mine come out during the day, most of the time I wouldn't see it for weeks, as it only came out in pitch black, even with food in the tank.
A good food source are live clams from Superstore, you can freeze then and throw one in every couple days, super easy.
All in all I'm glad I kept one for a while and got some pics and experience but I wouldn't bother with one again. Good luck.
