Hi Jack,
You would require as many floating licenses as you need for the number of computers you want to render from at any one time. Ie; if you have 5 workstations and you would want all 5 to be able to render at the same time, you'd need 5 floating licenses. However, if only two of the five needed concurrent rendering access at any one time, then only two floating licenses are needed. The beauty is, any of the workstations can send rendering jobs to the server, as long as only two of the five are doing it in this example. The licenses are controlled at the server so it's a simple setup. No muss, no fuss.
If you need more workstations to render concurrently, then you can incrementally add floating licenses as needed. This scales nicely and can help match your growth without overspending on licensing costs.
Here's a reference: http://www.indigorenderer.com/features/floatinglicences
As for SU Pro, I don't believe so. I checked the Indigo site and a few others but didn't find anything requiring SU Pro. The Skindigo exporter plugin works with both free and Pro versions. SU 8 IS required however.
The reason I was suggesting just getting a dedicated server is because I have experience in building a rendering farm. Some years ago I built a farm that used POVray over a network using Erlang. At the time I was learning the Erlang language and I had always wanted to build a rendering farm. While the project was successful, it required a lot of tweaking and such to get working decently.
Personally, I think Indigo's network rendering solution is robust, efficient and would give you a very good ROI.
Cheers.