Depends on what you keep ... If you go for Goldfish & other "fat" fish, going closer to 10x turnover is better ... ie a 500-600 gph
pump in your case.
A small sump might have problem with evaporation. By the time you factor in the headroom needed to contain the water from the display in case of power failure, you have maybe 5 gal
of water in the sump. You might have to top that up every day or 2 with evaporation.
A bigger sump is less maintenance. If you have room, it won't cost too much to get a 20 or a 30 gal
rubbermaid bin as your sump. If you want to try the 10, I would suggest making a tight fitting lid for it (and a good glass top for the display) to minimize evaporation. <And if you are doing CO2
for the plants, you will need that so you don't outgas all the CO2
in the sump.>
In addition to the bioballs, etc. mentioned, also consider polyester fiber batts s for your filter material.