The weight difference is very minor. The 3101 actually have more torque than the HS-55's. Using the HS-55's for the ailerons, then the 3101's for the elevator and rudder, should be fine.

You will not be wasting your money necessarily. The 7.4 volt LiPo will work with the brushless setup, but the 11.1 volt LiPo provides a whole lot more power, and the plane will perform much better. It would be worth it to just go brushless and go straight tot he 11.1 volt LiPo.

If you just want to fly with the stock setup you could just get a NiMh. Here is what I use http://www.batteryspace.com/index.as...ROD&ProdID=850 You will get about a 7-10 minute flight time. You could get the 2S LiPo, and get a little longer flight time, because they are lighter than the NiMh's, and then you could get a higher mah rating, but it is a lot of $$ for a batery that might not do you a lot of good if you go to brushless. And at 7.4 volts, the 8.4 volt NiMh will actually give the motor more power.

That HiMax is an inrunner. I would recommend an outrunner so you don't have to get a gearbox, they are just easier, no gear stipping, less parts the better. Something like this:
http://www.e-fliterc.com/Products/De...rodID=EFLM1400