as Skeptic pointed out, it all is covered here in the forum and has been asked before.
4. From your experience what is are the expences I am going to face to equip a flat or a house ? Lighting, TV/Stereo control and maybe shades and garden appliances ?
From my point of view:
Time=expense, to me time is the part I lack the most..
Hardware running the Core. Try to find what the users here report is working. Then when finding something. Do research in the forums for that spec. hw..
I cannot recommend any of the hw I have since the first motherboard I bought still does not seem to work with lmce(MSI K9N2GM-FIH) and the one being used now(ABIT AN-M2HD). well I am not sure..
It all comes down to what you will use(hdmi, dvi, spdif or coax for example) My experience is that you should count on having to work abit to get it all working. Hopefully it will all go smooth but as with all tech. stuff the easiest thing may be the hardest to achieve, if you are as me, a learner..
But, if you dont want to struggle or spend time on those things you can always find ready made systems from suppliers also found in this forum.
I use Zwave to control most of my lighting, mainly because it seemed to be the thing that was being worked on and updated most often. But it's not cheap..
Serial for the receiver onkyo 906, serial to usb adapter = cheap
IP Camera panasonic 111 something.. fairly expensive.
Possibility to back all the config and movies up, well then harddrives and an extra nodes to run as backup machine shall be counted in..
I would recommend to start off by setting up the core(or hybrid) and play around, Before you plan/buy the other hw. You will find the answers then and also new ideas will pop up.