Go BIG. If it's just a one-time thing, rent one. Otherwise you'll be spending a couple hundred bucks.
I bought the smallest submersible pump they sell at Lowes, 1/3 HP I think it was.
Anyway, it really suffers when hooked to a long hose. Right now I can barely keep up with the rainfall. How's 400 gallons/hour sound? Not very good, huh?
If there's a way to shut off your skimmers and use your pool pump and the main drain, you're set. I was amazed at the flow when I backflushed my filter yesterday. My guesstimate is about 250GPM, out a 100' 1½" hose!
EDIT: Pump is 1/6 HP, and it died when I left it run overnight, sometime during the night. I stand by Go BIG.