there's a guy on ebay selling the poly pads dirt cheap, i got 50 for £5 and they do the job fine.
i didn't clean my old juwel much at all. i think i only rinsed the bottom sponges once in 6 months, and the top maybe 3 times. it depends how gunked up they get though. you shouldn't need to replace anything apart from the poly pad. i'd leave the carbon sponge in so it acts as biomedia, but get another one incase you need to clear up any meds