Hmm. Perhaps I should have made that clearer Alan - I was proposing LDAP integration for user accounts, and file storage on your existing servers, not the new mac server, which I'd envisioned as being there only to let you do the restrictions - basically, Snow Leopard Server has three main components as far as networking is concerned - Workgroup Manager, which despite the name is the bit that lets you setup domains and is the only one I can remember (it was christmas when I last setup a Mac network), a security profile manager which broadly does the job of Group Policy Manager, and a third one which I think is server manager which lets you configure the services running on your server, eg SQL, IMAP, POP etc, which you tend not to use once you've got everything running properly!
That said, you could quite easily setup a Time Capsule if you wanted to go all Mac, or some other sort of NAS box to store the files on which would give you more space?
The alternative, and what I'd suggest for 150 concurrent users (how big is your music department?) would be to buy a great big Mac Pro and a separate copy of SLS, and stack it with oodles of disk space.