Server spec for asp.net 4 ?

Hi,
I have a friend who's creating a website for me using asp.net 4.0

I need a server on which to put the site, but neither of us know anything about servers. All I found out so far was that I need a Windows based server and not Linux, due to the asp.net 4 code being used.

Could anyone tell me anything I should know to help me get the right software \ server combination, please? Recommended companies (in the UK if possible) are also welcome.

The site will initally hold 10,000 wav \ aif audio files and matching mp3s (I own a music licensing company) so I know I will need a minimum 500Gb drive with another 500Gb RAID mirror. I will need something like Cpanel so I can create email addresses and track disk space and data transfer amounts, as the catalogue is expected to reach at least 20,000 tracks in wav \ aif and mp3 format.

Clients are companies who make tv programmes, so I need a dedicated server with enough cpu speed, ram and transfer rate to handle 1 to 50 companies possibly logging in at the same time, all playing mp3 files using the website, then downloading 10 to 40Mb audio files once they find a track that meets their requirements.

As far as cpu, ram, os and additional software I have no idea what I would need to cope with the above.

Thanks for any help - Adam!