I've recently started doing system administration for a very small ISP/web hosting company. Our hosting business is on the order of 150 sites currently, all managed manually with no control panel type application of any kind except some god-awful thing called PostOffice for mail accounts (strictly internal, not customer-facing). Accounts are currently managed with a small collection of shell scripts and all customer requests are handled via phone or e-mail. I would very much like to move over to using some sort of control panel, both to consolidate things for us and also because it's what customers want (I, for one, wouldn't buy hosting that didn't give me some direct control myself).

I've looked over the web sites of some of the more popular ones and one thing they're all lacking is clear, detailed, technical information about how they work on the back end. What I would like to find is a web site that compares the operation of some of them and give me a better idea of what might meet our needs best.

My specific questions are:

1. Will we have to run the panel or some sort of daemon on every server (web, mail, dns, etc)? I assume the answer is yes, as I don't know how central management would work otherwise.

2. Will we have to make major changes to how things are set up currently? I assume most pre-written packages will require things to be in certain places, web sites here, user accounts there, and so on. How flexable are these things generally? Are we going to have to rearrange everything to fit with the panel developer's vision of how things should be set up?

3. How are user accounts usually handled? In a database, or will accounts be created on every system running the software? How difficult will it be to add servers once we're running something like this?

4. How difficult is it to integrate one of these systems into an existing network? Once we pick one, are we pretty much stuck with it?

I realize, because of the number of different panels out there, there's no one answer to any of these questions. What I'm looking for first is a basic understanding of how these systems work.

A little background on our system so no one recommends anything completely inappropriate. We're currently running Debian Linux, Apache 1.3, PHP/MySQL, Sendmail with heavy spam filtering, Courier POP/IMAP with Maildir, Squirrelmail. We have six servers in production currently. We're using something called RT for support tracking, and some ancient home-rolled application for billing. We're looking for moderate to low cost, easily understandable licensing terms, and nothing that needs to phone home periodically. I'm not really looking for specific recommendations until I have a better understanding of what we'll be getting, but will accept them graciously.

Any help would be extremely appreciated. Thanks in advance.