I'd be cautious about going down the ASP on Mac road. It's not exactly industry standard.
There is a BIG difference between MacOS 9 and MacOS X (in one line, X is Unix, and 9 isn't) so find out what your client uses before you get too deep in.
Since you're not a Mac shop, you'll also want to steer away from any "Mac-centric" solutions like FileMaker Pro and Lasso.
You might be more interested in PHP, which can be used on Windows/Mac OS X/Unix, and is very popular, and easy to use with MySQL (which also runs under PC/MacOS X/Unix).
Otherwise, maybe talk your client into letting you host his site?
Just my 2¢.