uhm. i would have to ask how many people are you going to actually host on this server. that is the answer to your server question.. and you need to have high bandwidth (make sure it is a high upload, not just down speeds)
but then you have to think
you are the administrator of this server. are you going to use microsofts iis, or maybe apache? what one do you know better?
now you are in charge of the security of this server. if that customer gets hacked, personal data is compromised, or lost for good, you are responsible.. if your site gets hacked, and someone embeds a virus on the site, you better have insurance and alot of hours available to deal with this mess.
there is alot of things to think about, but with a server - you need to have an idea of how many sites you are actually going to host.