Hi SC_3D,
The main bottle neck might be the network among the machines after a while. If they don’t get data… However:
To say anything in general, without even knowing what kind of processes you will run, seems not really professional. How much needs to be transferred to a single machine, and how long will it take for the frame to render, how large are the final files? All questions you might not know now. To take only these three “variables” into the mix allows already for many theoretically results. Yet alone if you render single 8bit/c jpgs or 32bit/c float full multi-pass formats will change the situation dramatically. If you don’t plan for a feature film, the situation might change on a daily basis even.
You might check out a little bit how VFX studios these days set up render-farms, and what technology they use. Check out what Mike over at http://www.fxguide.com shares and will share. Search Render + Farms for example.
I would always invest in the real (current) need, perhaps a little bit below if possible (which keeps the people on their toes to be efficient), this leaves hopefully healthy budget for new tech in your pocket. Cash-flow is important to survive.
If you have the option to pick, stick with one CPU model or one vendor, at least that is what I got over the years, perhaps the support might have deeper insights.
Good luck
Sassi