The sand bed is a bad idea for a sump, IMO.
Fourth pic: That's possibly a stand for inside the sump? Ya have to sort of move it around to see where you think it fits and take another couple pictures.
In the first pic there's a couple slots on the far left of the sump. Those slots are probably for the mechanical filter. It's basically two panels with holes in them with some filter pad material sandwiched between and then the whole thing is slid into those slots. Water passes through those panels and is filtered by the pad. I have this on one of my sumps and it's very easy to maintain, I love it.
Quick question? Where's the pump? I didn't see any holes for an external pump? Are there any holes or do you have a submersible pump?