TM is really intended as a single-user backup solution, not a situation such as you describe. What might work better for your Mac people is for them to use TM on their individual machines to backup to their own external drives. However, that probably is not a feasible solution.
To add to the problem TM will not backup to a Windows server that does not support AFP protocols. You could set up some macros for Retrospect to ease the pain of retrieving data.
In any event if you are going to attach a 1 TB drive to the Windows server, why not simply attach it to the Mini? You can use TM to backup the mirrored RAID to the 1 TB drive. TM's overhead isn't that hefty.
I agree with Kappy. If all you're trying to do is make file retrieval easy, then you'll be fine with time machine.
Note that time machine doesn't back up databases. Things like the email database, MySQL, and most importantly, Open Directory.
Do not think that you're backing up your complete server with time machine. It will not restore your server whole.
As far as overhead, I have time machine working with about 900 gigs of user data. It runs its backup every hour for about 4 minutes. My users haven't said a word. Here's an image of the server graphs. You can see that time machine blips for a short time, but it's totally reasonable. Those blips are taking place overnight. During the day the server continues merrily along without interruption at all from time machine.
Kappy: The 1TB drive would be connected directly to the Mac mini, not a Windows machine nor over the network.
I should probably also mention that the Snow Leopard Server is only doing file sharing; email & all else are handled elsewhere.
Thank you gracoat; that information is exactly what I was looking for, some assurance that running TM on the server wouldn't slow things down.
When I moved from Windows 2003 server to Mac mini server 450GB of data and, when all was in place I attached for the first time to the mini the 1TB TM hard drive during working hours, the 25 users didn't notice anything at all.
The mini was under pressure for about 4 hours but as I said no noticeable slow down for the users.