For information on how to change your OS, please see the Knowledge Base.
Yes, that would be our Developer plan. It gives you the additional flexibility of choosing an operating system: Ubuntu, Debian, or CentOS. If you are not sure which plan will work best for you, our Sales Team can help you decide.
The Dedicated Server is a 64-bit environment.
Yes. You can use iptables on the container, or use the Power Panel to configure iptables. If you have DV Dedicated, you can also use the Firewall module in Plesk or Security Policies in cPanel®
Absolutely. Although (mt) Media Temple cannot assist in the installation of third-party applications, you are free to install and use whatever software you wish.