I personally wouldn't reccomend an ACD or GSP. Both need TONS of exercise. They are very active breeds that thrive on physical stimulation.

I think if you just want a dog that looks scary when you are walking down the street, you can get a german shepherd or a mastiff or a rottweiler.
I find GSDs and Rotties both very protective breeds, not just intimidating by looks -- even though they are big sweethearts. I've owned both breeds, and if a stranger ever entered my property, the stranger would be in big trouble. I've known alot more rottweilers that have absolutely no guard skills than GSDs though. I have never met a GSD that didn't have good guarding skills, whereas with rotties it's half and half.