gumtree - and deal with a gum nut lol... no thanks.... buy new - you have a warranty and you know exactly how the filter has been treated since new...
one other bonus with 2 cannisters -> you can clean them few weeks apart and also helps to prevent a bacteria crash by over cleaning your filter media which can happen sometimes.... plenty of reasons to run 2 filters
as for size - do it the easy way tell the shop where you're buying the filter(s) from, the tank size and what fish stocks you will be keeping in the tank... and if the shop owner knows his stuff he'll recommend the correct size for your tank.... although some research before hand isnt a bad thing either... going on your tank size i'd be looking for something in the 1000 l/h size... x 2 and also do some research on your shop where you intend to buy from... personally i'd avoid chain stores as most of the time the shop owner isnt even there... so the care factor IMO isnt the same... happy shopping