Servos - I would use lighter servos than the 300's. They are cheap ($10 per) and strong but big and heavy. I would use 81's or 85's. They are more expensive $20-30 apiece.

Battery - Most speed controllers plug into the main (and usually only) battery pack. The output plugs into the motor and also has a line that plugs into the rcvr. If you get NiMH batteries make sure your charger can accomodate them. I would get the NiCad pack for your first setup. They have a reputation for reliabilty and ruggedness.

There are a lot of electric sailplane fliers on the forum. Maybe we will raise one for some expert help.