I think where I'm at is trying to figure out what the main limiting factor is here versus building something from the ground up, piece by piece (probably PC based for cost).

Do you have a sense of specifically what the networking issues were?  For cinema rendering, my understanding is that once the net render files have been distributed amongst the nodes, their really isn't a ton of network traffic, except for the rendered frames being copied back over to the server.  However, I may be wrong on this… I don't know a ton of what goes on under the hood of these farms…

Have been exchanging emails with the owner of this company:

to see if he's heard any anecdotal info from any of his clients.  He hand't heard anything specific other than describing one server set-up he'd heard of that utilized 160 mac minis for some type of test automation (not sure what that is), but they were all being taxed 100% of the time and seemed to run fine…