1. The specs. look good for a first setup. If you plan on extending the system with any media directors (MDs), you might consider changing the hybrid to a core to ease the load, but I would not worry about that just yet.
Be warned: the HVR-4000 is not supported by default in LMCE. There exists a driver, but you will need to compile it yourself (See the www.linuxtv.org site for more information). On the other hand, I do use this card in my setup, and it works fine once you got the setup right.
Sorry, don't know anything about the sound card.
2. You will find some forum posts regarding MKV. I suggest searching the forums, as I have not tried MKV files myself.
3. There is an option to install the dvdcss library which allows you to play DVDs. I did have problems with playing a few DVDs.
4. Dont know for sure, but since it uses the Symbian OS, it might be possible. Check the wiki for mobile orbiters. If not, the JavaMO might be a possibility, although it is now quite finnished yet.
Regarding blu-ray, there have also been some forum posts. Don't have a drive myself, so I suggest doing a search in the forum.