WAS 5.0 will not be released until 3Q02, but you probably want to get the Technology Preview anyway. Since IBM tends to lag on the J2EE support (WAS 4.0 is J2EE 1.2, for instance), they put out a beta-type product called a "technology preview". The one you want is "WebSphere Technology for Developers", which essentially a free download preview of what WAS 5.0 will be with a J2EE 1.3 base:
To answer your question about Solaris, WebSphere App Server does run on it, but if you get the new Solaris 9, you get the new J2EE 1.3 App Server built-in, so why use WebSphere on that platform? If you're just experimenting with the product, then Win2K should be fine, but for production work you really should be on a UNIX platform.
If you haven't been to the WebSphere Developer Domain before, look around there lots of goodies there.