Advice Needed Re: Rena Cube Tank

Hi
I bought a Rena 100l cube tank on eBay a couple of months ago - long story short - offered it to a neighbour for her goldfish.  She's just bought a new tank so given it back (unused!). 
It has a used Rena Flow 400 pump, a very long sponge (unused) and 2 possible carbon(?) pouches in the large 3-part corner filter area.  The seller suggested I replaced it with a small canister filter. 
Any advice would be appreciated please. 
Thanks in advance!

Thanks Sue.  I don't know much about filters being a relative newbie.  Are there any you would recommend for a 100l?  Money is tight but obviously I want to make sure the fish will be ok!  In my fluval edge there's sponge, carbon & biomax.  This fishtank only has the carbon & the foam so do I need to add anything else e.g. similar to biomax?

That's really helpful - thanks so much, Sue.   

Thanks fcmf - I'll have a look at the fluval filters too.