We use GFI Faxmaker with a 100 user license as we're massive fax users.
Works a treat. The biggest cost you'll see if you setup your own fax server is the ludicrous costings of the faxboards. Our Dialogic PCI 2-port ISDN card was circa £4k, but you might be able to get a cheaper option given the number of users you're likely to have.
Apart from receiving faxes, we use it a lot for sending out confirmations and purchase orders. Simply click print in the app, select Faxmaker as the printer and it opens up a new mail window in Outlook, along with the fax attachment. Simply type in the receiving fax number, followed by @faxmaker.com and away it goes.
D.