wheel - 12:16 pm on Jan 12, 2011 (gmt 0)
I would suggest that you don't do a colo, do a managed dedicated server. That way someone else looks after the hardware and operating system.
Hardware wise, get what you can afford. It doesn't look like you have anything that a routine webserver can handle. It's unlikely that you'll have to worry about disk size or speed unless you go cheap.
What you should perhaps focus on is getting the right support and admin software for the server. The biggest problem for newbs isn't getting the right hardware, it's figuring out how to do stuff (like add domains, mailboxes, check server load - all the server admin stuff).