I've started to look into LinuxMCE and I'm prepared to give it a try...
Just questions on system design, I want the possibility to play a dvd in the living room without running to the core to insert it:
is it ok to add a player in the Media Director to do this?
if yes, is it then possible to stream it to the kitchen using the follow me feature?
or can I integrate an AV receiver (eg. Denon AVR-4306) wich has ethernet streaming capabilities?
Sure you can... thats one of the major features of LinuxMCE. Your Core is the 'Media Server' for your home. So you can store your media on the Core's hard drive(s) (or on other storage around your home too... but live simply at first!). Then you can add Media Directors (network bootable media players) around your home and then any content stored on the Core can be accessed and streamed to any of your Media Directors. Media Directors need no internal Hard Drive (but can have one if you choose) they typically have just a motherboard, processor, RAM and an optional optical drive for playing DVD/CD's from disc locally.
My suggestion is start simply and then build out form there - LinuxMCE is a very powerful multifaceted system that can probably do almost anything you could wish... but it can be a little daunting to understand because of this!
See my companies Wiki page here for some example hardware to give you some ideas
Hope the above helps you.
All the best