Freebsd – cpulimit for FreeBSD

As long as you do not require a kernel different from the host system to be run in one of your guests, operating system virtualization will probably be the solution with the least amount of overhead.

Next to the kernel version restriction, you'll also find it impossible to access raw devices from within a jail, or to alter the network config from the inside. Since you have access to the host system, you can set that one up as you require, so this is not really an issue I guess.

From the description you give, I'd certainly recommend you to give jails a try.

The FreeBSD Handbook is your friend for all basic questions.

The ports tree and the pkg_* system are one in the same. There are also additional pkg_8 management tools in the ports tree if the built-in ones aren't enough.

Upgrading will depend heavily on your environment, but can be as easy as cd /usr/src; make buildworld; make installworld and horribly complex in the opposite environment.

The FreeBSD Security Team determines how long releases are supported (bug/security fixes). Scroll to the bottom of their page for the current schedule.

Same gotchas as most other *nix OSes; if you're familiar with Debian, you should be familiar with many already. If you have additional or more specific questions feel free to ask.