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Sept. is cool, not freezing but cool, sometimes very cool. You may want to bring clothes from the US as you will find some things are very expensive in Scotland especially given the bad exchange rate. Sweaters (wool) can be found reasonbly though.
Regarding restaurants, Gringo Bill's is fairly good except you will find french fries served with all the dishes. You probably won't find authentic Mexican food in Scotland, but try other types including Indian. You should enjoy your experience, just don't compare it to California. Make sure you go to the Jekyll/Hyde pub, near Gringo Bill's and see if you can find the restroom.