Well, knowing Linux would separate me from all the other server admins and allow me to offer much cheaper prices. I don't have to worry about licensing issues or how many users will be connected. It's also pretty easy to add a user or a network share graphically.
I (and the others, too) don't want to hit on anybody's sensitivities here. Being a Linux expert is a valuable skill and Linux certainly has a place in the business world. But it's a much smaller market than Windows servers or desktops.
It's also understandable to think of free Linux licensing as a huge advantage. Well, it's an advantage, but when you consider that the lifetime of a small business server is five years and that SBS Server 2003 licensing came to about $80 per User for five years...the cost of a nice lunch once a year. Compared to the cost of the server itself, backups, installation, configuration, and maintenance, the licensing costs for the server software are pretty small.
Anyway, whatever you decide to do, the best of luck to you. The IT business, at least where I live is VERY tough right now.
And, yeah, take a look at Outlook combined with MS Exchange. That's a VERY different animal from hooking Outlook up to an IMAP or POP server.