Wow, I am jealous, 600L tank to start with
Good luck with it.
As for the filter, if I were you I would be getting at least two to be on the safe side, may be one internal, one external, if not 3 As for the capacity of them together, at least 10x water flow per hour would be good, which in your case is 6000L/hr.
As mike455555 said, I would choose the type/color of gravel well, as it is a pain to change it after. It does depend on the fish you are getting, but sand is a good idea as most bottom dwellers prefer it.
And make sure you cycle your tank before adding any fish. There is a lot of info here on the beginners forum how to do it, and I am certain a general internet search will give you a good idea. That's only presuming you've never done it before
If you find it hard and prefer a smaller tank, I can swap with you
