A sump is a good idea on any tank size. It increases the amount of water volume you have so it's less prone to dramatic salinity fluctuations from evaporation, among other things. Sumps rule! And it gives you a place to hide all your crap like heaters and skimmers. However on a tank with live rock, I wouldn't put any bio balls in it. No point unless you like to produce nitrate. Take some pictures of the tank. Lets see what you have got.
