Is a GSD a good Agility canidate?

[Whisper]

For your first question, yes! GSDs are athletic, energetic, and intelligent. I've seen some great GSDs in agility. Certain lines, though, I'm not even sure how they walk, let alone jump, but that's a whole other topic.
I recommend the book "Control Unleashed." It's directed towards agility, but there are lots of confidence and impulse control exercises in there that are great whether you do agility or not.