Server Administrator to manage a VPS Server (Unix)

I'm looking into hosting options on behalf of my client. The server needs to be set up so it can handle the following features:

The ability for registered users to email other groups of registered users through a web interface. This works by cycling through the database to select relevant emails, then sending a copy of the mail to each mail address via the php mail() function. In reality this means we need the site to send out up to 100 or so mails at a time, with the occasional possibility that there may be two or more users doing this at the same time.

Other requirements we need are as follows:

Allow users to upload and write document and graphic files to the server (specifically using the move_uploaded_file() function)
Write folders and files to the server programmatically (specifically using the fopen() function)
Change folder and file permissions programmatically (specifically using the chmod() function)
Change folder and file permissions manually (using the hosting control panel or whatever)
Online database administration via phpMyAdmin or similar
Set up an unlimited number of subdomains

The ability to set up forwarding of multiple domain names to the various subdomains would also be desirable.

We envisage that a day will be initially required to set up the hosting with ad hoc maintenance required on a month by month basis.