I've had GSDs since I was 12 years old. I've had purebreds, and I've had mixes; I've raised two from puppyhood (the first one, Purdue, and Bimmer, the GSD/wolf mix I have now) and all but Purdue were rescues. Bear, who became a certified therapy dog, was a GSD/Akita mix and was one of the most intelligent and formidable creatures ever; he also knew his business and I knew I was utterly safe with Bear. If we heard something, no matter how late it was, he and I would just go out and see what was up.
There's something special about the rescues. They understand what you've done for them, and, when you adopt a more adult dog, you have a very good idea of the temperament from the beginning. Breed specific rescues are generally an excellent way to find a good dog that suits your personality and situation. Most rescue organizations not only check out their dogs, but take care to match them to appropriate homes.
You don't need an attack dog. Trained guard and attack dogs are only for specific situations and highly experienced, well trained owners. A fearful owner is absolutely the most dangerous owner for a trained guard dog. The dog will react to your fear, and the results could be tragic; not that it will turn on you, but your fearfulness can cause the dog to react far too strongly in the wrong situation.
Any well adjusted, well loved dog is going to protect you. A dog that is well trained in the basic commands is going to be a dog that you can handle in almost any situation, provided you have participated in the training.
In your situation, I would strongly caution you not to pick a breed like the Rottweiler, Doberman, or anything resembling a Pit Bull or American Staffordshire Terrier. That's going to draw the kind of attention you DON'T want especially that of the very punks and thugs you're afraid of.
I'll also recommend that you take some self defense classes. They will give you more confidence and make you feel like less of a potential victim; an attitude which will also communicate itself to whatever dog you eventually bond with.