Colleges accept either the SAT or the ACT, meaning that they're both seen as equal in value.
I agree with the above poster in that if you're motivated enough, you won't need to take an expensive prep class. Buy the Blue Book, Red Book, and a few other prep books for review, and you should be fine.
The SAT II Subject Tests should be taken after you complete the corresponding classes. If that's not possible, then just research reviews for prep books and find the best one for you. These are important only if the college requires it, otherwise they're pointless. A few colleges require three, but most require two. These are of secondary importance to the SAT/ACT.