I would recommend against the hang-on-the-back (HOB) filter if possible. Octopus produce significantly more waste than the average fish so they need a much more robust filtration system than a typical aquarium. Did you tell the person at BRS that you were planning an octo tank? Hopefully they know better... but I digress.
If it is possible and within your budget I would recommend buying a sump to place under the 55gal tank. The sump serves two main purposes.
1) it lets you put all the "problematic" elements of your tank as far away from your octopus as possible. Pumps and heaters and protein skimmers in the same tank as the octopus are all a recipe for tangled arms and dead octopuses.
2) It makes it significantly easier to do routine maintenance and cleaning without disturbing your animal.
Is your tank glass or acrylic? If it's acrylic then it should be no issue to get the tank prepped for the sump. Glass tanks are a bit more fussy unless you have experience drilling glass.
What is your budget for this aquarium? That will help me give more applicable suggestions for what you should be looking for for your setup. Primary things to focus on are:
-Sump to put all your pumps and filtration goodies in
-Some form of mechanical filtration (usually included in the sump as filter bags or filter pads)
-Some form of chemical filtration (usually in the form of a bag of activated carbon and other chemicals placed in or near a filter bag
-Some form of biological filtration (usually a mesh bag with your biomedia floating somewhere in the sump that has high flow)
-Protein skimmer (in your case, the HOB skimmer you linked will probably be just fine as long as you can devise a strategy to keep your octopus away from it, but an immersion skimmer in your sump will be much easier and more octo-proof)
-Heater or Chiller depending on what octo species you plan to get
-Temperature probe if it is within your budget
Here is a sump I have used in the past and can vouch for. I think that $250 is a bit steep and you might be able to find it cheaper somewhere else.
Here is a kit I found on ebay for $50 to convert a 15 gal glass tank to a sump. It looks good to me but I cannot vouch for this item. I'm a bit nervous about glass tanks in general (personal preference) but if you are comfortable this could be a more affordable option. I would make a lid for the sump to prevent evaporation.