any external filter at the right size will do there are still new 03 old stock being sold for real peanuts as the series are now old design-so you could buy and some spares for very little. As long as you get a bigger than the tank size min requirement you could do it very cheaply this way
I have a 203 fluval I've had for years(not selt priming) so you need to prime intake pipe first time you use it. The only thing I,ve replaced are the taps at £3.50 each as they go stiff with age and some clear tube. there are easier quick release fittings too.
they are easy to use just put a bucket under it turn off power and taps and undo. To restart open outlet first(release any air pressure) and then the inlet, allow to fill and turn on. if you can find one for say £20-25 pounds brand new then buy it or a secondhand or cheap 04 series-though I heard there were some shaft wear/noise problems when the shafts were worn on 04 series, I beleive the shaft/impellor was updated to stop this and avaliable as a spare part