SixShooterz,maybe I can answer some of your questions.
Solution:
Q1:You've got to choose
postfix as your secure mail server.I am using it and its ease of use,secure attract me.
Q2:This can be accomplished by installing
squirrelmail.This is a PHP based webmail software.Then,you can install related plugins to achieve your goal.
Q5:With the combination of
postfix & squirrelmail,you can use it as a webmail.By turning off related port, e.g. 110,you can let the user uses webmail only.
Q3 & Q4,I am creating my system right now.I don't know and I rarely hear that there are a system like that.Maybe you can google it.