I think he already has the pump, which is unfortunate as I don't think it's near big enough.
Rule if thumb is at least 5x turnover thru the sump, or 600 gph
min for your 120. Pump rate is based on the headloss chart. Headloss is measured as the hight difference between the top of the water in the return section of the sump to the water level in the tank. Plumbing locations don't matter here (ie where the outflow pipe is or the pump hieght). Add 1' for every 90 deg bend in the return plumbing.
Sicce 3.0 for 3.5' headloss plus 1x90 is just over 500gph.
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