Yosemite Server will allow you to use your Mac Pro as a Time Machine server (and a lot more) for all the Macs in your home for a lot less than the cost of another external drive.
The following is a decent summary of everything included with the Server app. (It's ~$20 on the Mac App Store.)
I'll start you off on the page dealing with Time Machine, but I recommend reading the whole thing when you have time...
http://arstechnica.com/apple/2014/11/a-power-users-guide-to-os-x-server-yosemite-edition/6/