In my opinion you would be better off not getting the sunsun and just saving your money up abit more and look at getting either a fluvel or eheim branded canister, I have heard bad things about the sunsun brand.
Also with the fish you currently keep in the 300ltr planting it out will not really work in my experience, not sure about the silver dollars but both oscars and plecos don't mix well with a planted aquarium unless you have more patience and time then Mother Theresa.