I currently have a box using ensim. I was wondering how much it would cost to add on Plesk. Do I need to remove ensim first? etc. Also, does anyone know what version of BIND plesk is using?

Depending on how much your Ensim server is used I'd be inclined to ask Rackshack whether it is possible they will wipe your machine and install Plesk on the server for you.

If you can't go down that route you will have to strip all parts of Ensim and the software it uses before installing Plesk on your box.

I believe Plesk v2.01 currently runs bind-9.1.3, v2.5 of Plesk will upgrade this to bind-9.2.0.