What kind of data rate would you expect to be pushing? I would say about 128kbps as a minimum, and 100 of those, well thats 12.5 MBits/second.
Seeing as your going to be suppling a large amount of viewers at one time (100+) your going to need to be on the safe side for bandwidth, your going to need about 6.6TB quality bandwidth on a 100mbps connection, or a dedicated 20mbps connection (Not share unmetered).
You are also going to need a powerfull server to push that much, your going to need a large processor (2.8GHz Minimum, Dual Core would be best), and a large amount of RAM (2GB minimum).
Overall, this venture is going to cost around $400/month for the connection, and around $200-$300/month for the server, totalling out at $700/month.
We at galaxy-solutions have a single client using such a system to host a TV stream, and it works out quite well, but instead of the 20mbps you need, they have 50mbps.
If you want, I can knock you up a quote.