First off, don't buy from the campus bookstore. The prices are rip-offs and the books can usually be found online on sites such as amazon for much cheaper prices. If you're registering at orientation, you might not get your books in time for the first lecture or two, but I do believe as a washu student you qualify for expedited shipping? Either way, not having your books the first week likely will not hurt.
If ordering online isn't an option, check out lock and chain. This is a student group which, among other things, arranges a market where students can buy and sell books for fairly cheap prices. (Along this same line, if you know students already at washu, see if they have any to sell you).
As for the health forms: I have no idea, sorry.
Edit: as to registration, most freshmen don't have any problems. You can get into almost any gen Ed class, albeit not necessarily at the time you may prefer.