Personally, I'd start with a pup from an experienced breeder who is very hands on with litter.
Ask about recommendations and follow up. Ask about health care. Ask how many litters they've bred and ask where these dogs are now? Are any of them training in Schutzhund, or other disciplines? Are his dogs titled?
Just the presence of a GSD will probably be enough to keep most folks at bay.
Wouldn't want a dog someone else had trained unless I had absolute faith in them and many, many references.
I think a pup will bond closely with you and your child and, if of stable temperament, I doubt it will be affected by your Aussie. Of course, you'd have to wait a bit for maturity.
Also, where are you located? Is there a Schutzhund club there? If so, call and ask to visit and talk to those who work their dogs.
Don't be shy.. this pup will be with you for many years.
Nothing is as simple as it seems or as complicated as we make it~
Attitude is everything -- Pick a good one!
Sofie AKA Ussina vom Haus Brezel
Jack von Jagenstadt
Tatty - Burmese bad cat