Well, having actually worked with my wife's GSD rescue untill she retired from it. I can tell you that there are good and bad rescue groups out there. Good rescues can be an advantage as they well be able to tell you what the behaviour of the dog is like ie. good with kids, cats, strangers and so forth and so on. You are also getting a dog that has had any medical treatment it required, usually at great expense to the group (I know). And you don't play the guessing game of what the dog will be like when it grows up, or if it will have development issues. The dog will have a life time garauntee that if you ever decide you don't want the dog, it must come back to them.
Bad rescues will let you take the dog the same day you see it, without doing anything to determine if your household is right for the dog, or if you and the dog are a good match. It may have come straight from the shelter it was rescued from. They may or may not take it back if there is a problem. Like say, you discover that it's people aggressive. That's happened with a group we know.
Btw, you can get Pups from rescues as well as adults. Just depends on what they have at the moment.
As for the bonding. Forget that. Rescues will bond with you just as strongly as long as you do the work to earn it. Bonds of any kind are forged, they don't just occur because the dog is young and stupid.
I'll agree in a limited fashion on the training. You can give the dog to someone else who will board, feed, and train the dog then return it to you. Bad idea, because the dog has not learned to respect and obey YOUR commands. There's a lot of training of the owner that gets done in a good class btw. It's not just about training the dog to obey no matter what. That's done with negative reinforcement and leaves you with a scared dog that won't take any initiative at all, and certainly won't "bond" with you.
OTH, there are a lot of bad trainers out there, so you'll need to do some research and check references. We can give you the addresses of some internet dog boards if you like and you can see if anyone can point you to some good places.
Mixed breeds: As long as your adopting from a good rescue (and there are rescues that take mutts) it will have spent enough time living with a foster home that you'll know what it's traits are. Judging from the mix it is supposed to be made from won't work, as it's frequently just a guess anyway. The shelter we used to work with had a "policy" that's pretty common. If the ears point up, it's a German Shepherd mix. If the ears hang down, it's a Labrador mix.
There's now way to check geneticaly, so you just eyeball it.
We fostered a pregnant bitch that looked like some sort of GSD mix. The puppies looked like purebred Australian Cattle Dogs. Even ACD breeders told us they would have assumed they were pure if they hadn't seen the pictures of the birth. You never know! Here's another example from my wife's web page. Take a look at Carmel and her pups:
http://www.rawdogranch.com/fosters.htm