Possibly Adding sump to 20 gal Nano, Advice

As the title states, i am considering adding a 5-10 gal sump below the tank. I am mostly doing it to move most of my equipment out of sight. I wanted to put my heater, skimmer, and phosphate reactor with the sump. And possibly my ac 70 cheato fuge. I was looking at doing a hob overflow box to siphon the water to below. I can only go with the 10 gal after I take some measurements and know for certain it will fit in the cabinet with my 5 gal ato system.

So my main questions are:

which overflow box should I go with?

which return pump?

would a 5 gal be a waste of time aka would all my equipment fit?

how do i ensure no plumbing disasters such as an overflow etc?

and anything else i should know about?