IMO, you should contact an ISP that can provide you with Internet access in all 3 buildings before buying anything. It will make things alot easier in the long run to get service to all 3 buildings from the same provider.
No matter what, you're going to be able to get T1 access regardless of your location, which would be crucial at least from your office if you plan on pushing out streaming video regularly. In fact, that may not be enough bandwidth.
As far as the other two offices go who will be receiving the stream, it's a tough call because you didn't mention the size of the building/number of Internet users. Maybe you can get away with some type of business grade dedicated loop ADSL or SDSL. As far as reliability, you get what you pay for. Time to repair on those types of DSL services is generally in the 24-36 hour range, whereas a T1 circuit of any type will be more like 4-6 hours.
If you're looking at many users, and want reliable and secure broadcast, then you should really investigate point-2-point T1s to connect your offices.
I'd strongly recommend that you find a good consulting company in your area and contract them. You posted an expensive proposition and you're probably not going to want to try and tackle it on your own.
