RE: The right Li-po?
Use the prop that came with the GWS kit. I believe it came with a 9x7 SloFlyer prop and a 10x6 HD prop. Either one will work.
Most other Himaxx 2015-41XX applications I've read about run a 3S LiPoly pack. That is, three cells in series, and if I recall correctly, the current runs right around 12 Amps or so when you swap it in in place of the stock 300 can motor.
First and foremost, you'll want a pack that can handle 12 Amps. LiPoly packs are rated in terms of C, or capacity. A pack rated at 10C with a capacity of 1200mAh can handle 12000mA, or 12 Amps. There are plenty of packs on the market that can handle 12 Amps. The Kokam 1500, Great Planes Electrifly 1200, and others.