Conor, you're absolutely right about the ISP restrictions. I know both Verizon (FiOS and DSL) and Comcast permit you to run servers on their restricted ports (80, 25, etc.) as long as you are on one of their business plans and I'm sure that goes for most other residential serving ISP's as well.

However, IGreGzI if you're really considering a real data center in your home (instead of a server in the corner of a room) than you really need to consider a dedicated circuit from a provider like Verizon, AT&T, Qwest, etc. Like Lesli said, if you're going to be running these servers as a service for any possible customers, than you need to have a quality setup.