Launchd literally brings up the operating system. It is a replacement for older things like init, rc, cron and so on. You don't have to log in for launchd to do it's job; just pushing the power button on your Mac is sufficient to get launchd going.
LaunchDaemons run as root, while LaunchAgents typically run as a non-root user. You can make the decision of which you want to use. Once you do, simply write a Launch* file and put it in the appropriate directory. You can manuallly load it with launchctl, or just restart and the system will load it for you.
Reading your reply, however, it sounds like you're more interested in convenience than security. You also want to run iTunes, which has a GUI. In that case, save yourself the hassle of doing any of this and just turn on Automatic Login and drag the things you want to your 'Login Items' pane.
You are correct, security is not a huge concern.
However I do need the server to start up processes if it goes down for some reason or has to restart itself. The VM of server 2008 will have the important stuff in it and that will be protected, but that has to come back up. If the UPS shuts down the server due to power loss I need the VM's to start up on reboot, of course iTunes and the rest would be nice too.
For iTunes all I want it to do is launch so we can share. I so wish Apple would come out with an iTunes server. We have 2 Apple TV's and looking for the convenience of always on.
Same with our Aperture library. I am absolute shocked at how fast Aperture is over the network, more shocked because the new server is my old Mac, faster over the network than it was local. I haven't even put the RAID array in. Now if it was only multi-user....
I've learned looked at the general problems with Apple's multiuser software. Apple refuses to support mySQL internally and they lack a solid database system. Everything and the kitchen sink runs on SQL Server, and works well because of it. I doubt Apple would ever use mySQL as their backend but would solve a lot of multi-user problems for their apps.
Thanks for the info, I have some reading to do.