If you can solder, you're in business...
That pack would work, but this is how I generally get them. I do try to stay with a state side vendor:
http://www.ebay.com/itm/A123-Battery...item1c53b982f9
Leads from dead servos, as long as the connectors are ok, can be re-purposed as battery power leads, or you can buy new easily.
Parallel circuit, exactly. Not only complete power redundancy, but double the amperage capabilities - potentially an issue once you start running planes like you're talking about using powerful digital servos.
Optical (ignition) switch = switch that allows remote control of the planes ignition via your transmitter (many use the landing gear switch). Not just for safety, but because they're darn handy. We can spend some time here if you aren't familiar. Just holler.
No clue if or which might have shipped with an AGM. -Al