some basic explanations to a new user: debian

[drein]

Hello,
I'm using a vps from a week, so I'm not very good with linux.
I have plesk, but I have some problems with it, because I am visual impaired and plesk accessibility is very low.
so I'd like to install this panel, it seems easy to use!
I read the documentation where you explain how install isp in debian, I have debian squeeze 64bit, but I see many steps... my questions are:
1. Can I install isp in a system that is already configured for internet, with 13 email, 1 domain, 1 subdomain, etc?
2. Since I have already plesk installed, does it means that I have all the prerequisite to install easily isp?
I use apache with mysql, and the website is made in drupal.
Thank you for your patience.