If you're content with your current hardware other than the drive space issues, why not add more drive space?
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An enterprise that size can get away with a $1-2k server depending on options and warranty. Maybe even significantly less depending on what's on sale, etc.
You can re-install Server 2003 legally as long as you remove the old server within 30 days.
I would not recommend UNIX for this enterprise. I'm assuming that you're already in Active Directory for the current setup? If not, I would setup the new enterprise with AD.
I'm neither licensed nor bonded but I have been working in this kind of environment for many years with an extensive resume to back it up. I'll be happy to discuss your current setup or even have a look and discuss a quote. Feel free to PM me.