yes you can buy these before knowing what the prof will assign. Your classes will be in very basic areas- contracts, torts... Study guides come in each of these subjects. They won't cover exactly what is in your class but they give you the general idea.
A commercial outline is essentially an outline of the black letter law you will need to learn in the class. I find them uniformily unhelpful- mostly because I do not learn well from outlines. Other prose study guides I do use, mostly Examples and Explanations series.
Yeah knowing some black letter law going in can't hurt, but I really would suggest that you just use this summer to relax, especially if you are not working. Law school will take up a ton of your time and thoughts once you are there and you will be absolutely wasting your summers if you don't have legal jobs during them. Therefore, you may want to use this time to relax and prepare yourself mentally and physically for law school. By preparing yourself mentally, I don't mean try to get a head start on your classes, but rather clear your mind, enjoy your summer, and be ready to hit the ground running your first day.