Since you already have an HOB I would go with the Canister. Personally I would go with the 2217 because it moves 264gph (Gallons per hour) compared to the 2215 which is 164gph. I would go with Pool filter sand over gravel. Much easier to clean on more to their natural environment.
As far as the heater, try using the old one first. You want to go with the smallest wattage possible incase the heater breaks and fries your fish. So use that one to see if it does the job, if not you can upgrade to a larger one.
As far as the heater, try using the old one first. You want to go with the smallest wattage possible incase the heater breaks and fries your fish. So use that one to see if it does the job, if not you can upgrade to a larger one.