Yep, the best 'filter' in a salt tank is live rock. It's the natural filter in the ocean, and with the increased popularity of it's use, people have been able to keep species of fish that, 30 years ago, were unable to be kept alive in captivity. Now some of these species thrive because they're in the correct surroundings.
Instead of spending $100 on a nice canister, put that towards live rock.
As for your tank size, the bigger, the better.
Former advisor and planted tank geek...life's moved on though.