The server hardware requirements are very light. Even a 4 or 5yr old PC is a good candidate for a dedicated server. Have an old PC (or PCs) laying around you can scrounge for parts? If not, what budget are you working with?
The server bandwidth requirements are a bit higher. You'd want a fat internet cable connection for a 24 person server.
Have you looked over the
Running multiple instances of TF2 is possible. The limiting factor will be the upload bandwidth of your internet connection.
With a fast cable connection anything more than 1x 24 man busy game will saturate the internet bandwidth.
It's possible you could use 2x 12 man games. For a DSL 3GB internet connection a busy 12man game would saturate the internet connection.