I feel your pain
I tried that approach before on Snow Leopard, X Serve and an external sata raid from OWC computers. Running OSX Server it was the server for a workgroup. It worked very well as a Time Machine Server. Out of all the experiences (freenas, drobo and OSX Server), OSX Server was the easiest solution. Certainly the mac mini and thunderbolt enclosure would be an amazing solution.
I decided against that purely in my case because of cost and that I’m paying directly for it, but it would be the best solution.