Hello there Nosnam,
Here are the answers to your questions,
to the best of my knowledge 
1) Are there customized packages that can not be updated without breaking the control panel?
That depends on what packages you are talking about. Plesk relies on PHP and MySql, so you have to be careful what you do with those, but for the most part it's fairly independent from the other packages that you may want to customize.
2) Does plesk have the security of ensim (Admins of domains are not able to browse the contents of other domains at all)
Plesk 7.0 does not do this by default, although I just read that there is an option to allow chrooted shell access in Plesk 7.1 Reloaded.
3) Does plesk come packaged with MySQL or do all users/databases have to be added manually?
Plesk itself uses MySql, so it does come installed on the server. There is also an interface in the control panel to limit the number of databases a user can create, allow users to create databases, and control user access to databases. Phpmyadmin is included for when more functionality and flexibility is needed than what is offered in the control panel.
4) Does plesk have a reseller control panel?
Yes it does. Before you can create a site in Plesk, you must create a client. You can then give clients access to log in to Plesk and create and manage their own domains in the client control panel.
5) How much work is involved in setting up a firewall, intrusion detection (Tripwire, snort), and spamassassin .
That depends on what you want to do, and how tightly you want the box secured. There are several fine how-to's listed here on the forums.
6) Is there a domain limit per box?
The domain limit is determined by the license. Plesk for Linux comes with a 100 domain license, which can be upgraded to the unlimited license if needed for a one-time licensing fee.
7) Ensim puts one hell of a load on my server, at log rotation times. Does plesk do the same?
I can't speak on that one. I do know what you mean though, I used to manage an Ensim server and saw the load spike every night at 4AM. I have Plesk server now, but I haven't had a chance to set up the monitoring scripts that would give me the answer to this question...
Plesk does automatically rotate all logs, right? Or am I going to have to set up cron jobs.
Yes, Plesk does rotate the logs. The cron jobs are already set up by Plesk to take care of that.
9) Does anyone have experience with the Plesk staff (Handling of trouble tickets and such)
Plesk is very fast, efficient, and helpful with every request that we have ever made. Most issues can be handled without their intervention, but when something does go wrong that requires their expertise, it is always resolved in a timely manner.
10) I read somewhere that it costs money to upgrade plesk to newer versions, is this true? If so, is there legacy support for older versions, to ensure that they do not have vulnerabilities, forcing you to pay for newer versions.
Some version upgrades do cost money, but we try to keep those costs down as much as possible for our customers, usually by absorbing that cost ourselves. If you check the "Services" section of our web site, we are currently still offering the upgrade to 7.0 free of charge to our existing Plesk 5.0 and 6.0 customers. Patches are currently available from Plesk dating back to 5.0.
I hope this helps. Please let us know if you have any further questions.
Thanks,
- Joe