Hello all you reef geniuses out there!

Help me help my dad design a new sump for his tank.

He currently runs a 55g Clarity Plus with a hang on overflow box made by an company that went out o business awhile ago i guess. This flows down a flexible hose into a bioball/ polyfilter box which sits and drains into a 15g AGA which then houses a UV sterilizer and protein skimmer (which needs to be replaced with one that skims...) and the Quiet One 2200 return pump. He also uses passive chemical filtration as the media just sits in the AGA.

If you were starting from scratch what is an easy/ effective sump design/product you recommend and what skimmer do you like if the biggest issue is how well it works and reasonable cost?

PLEASE school me on sumps as ive never had one really...

Should he look at "Berlin sumps"? Maybe get a 20 long AGA and build in baffles and ditch the bioballs for some more live sand and rock???