reccomended protein skimmer for 180g plus 55g sump

Whats the reccomended protein skimmer and size for a 180gallon system with a 55 gallon sump tank. I am on a budget so I'm not looking to buy the most expensive one, but I would like a good one for the money. Reef Octopus looks like it's in my price range, but I do not know what size I need.