Setting up Server on a mac mini? or my mac pro?

Mac mini

I would certainly go for the mac mini, for the only reason that you work on the other machine and there is less chance of any upsets to the server if no one works on it.
Any server uses quite a lot of resources memory, cpu. Another reason not to use your main work machine.
I would max out the ram on the mini as well this will make a difference.
The ten client thing I don't know.
By the way Mac OS server is simple to set up.
Regards.

I ran low usage Tiger Server on a 1.66 Dual Core Mini with no problems. It was on a small in-house network of three macs, most using wireless-G networking. It had the same client-side response times as the Xserve I was managing at the home office location. It also hosted a low-usage web site as well, again with reasonable external response times. (When I upgraded the mini to Leopard Client, with no other hardware changes, file copying times alone increased significantly.)

I don't know what web bandwidth you expect, but for local file sharing alone, the mini would likely suit you fine as a server platform in your case. If not, it's quite a reasonably priced experiment.

I'd advise against running server on a production machine, as it limits available resources to the local user. Plus, on the off chance you need to restart your machine, you need to be aware of connected clients.