Qwest's 1st Rule of Bandwidth Caps: Don't Talk About The Caps
You also need to check the fine print of your service contract to see if they allow you to run a server on a standard broadband contract. I know you probably don't think of 'streaming' video or audio as running a 'server' but that is exactly what you're doing. You might need a business class broadband plan to allow you to do that.
The actual hardware to compress and push the content up to JustinTV isn't a major consideration. Any fast dual core Sandy Bridge CPU tower will do the job nicely.
$500 if you already have a monitor.