You could get away with a rio for that small a tank. Like I said I use a rio 3100 that flows 999 GPH with out any kind of head. @ a 6ft head if I remember my math correctly the pump should flow 250-290 gph. That will turn my tank 5 times an hour. HTH
That depends on how large a tank and sump you have. I use a rio 3000 for my 58g but that wouldn't be enough for a 120g. The more flow you can have without draining the sump dry the better.