Homeseer is very good. I have beta tested their software in the past.
I would go the build your own pc route. And only use it for Homeseer. I did find that a pc running Homeseer and media server software (I used to have a pinnacle showcenter ) wasn't a good idea.
As regards to sound Sonos is excellent. Very flexible.
If you are going to use a Sonos based system then you may want to store your media on a NAS type device.
What do you want to do with the lighting. Just turn it on or have the ability to dim it or have set scenes, presence detection, give your house that lived in look when you are away.
Voice control is very hit and miss and you will need quite a powerful machine to do it well. Also in my experience if the command isn't said exactly the same way and with the same level of background noise you start to run into problems. so you will really run into problems when the TV is on. It is getting better, but still has a long way to go.
If you are on a limited budget then I would do it bit by bit rather than all in one hit.
You mentioned X10 and Z-wave. X10 is OK but it can be a bit problematic, and requires a bit more hands on testing and sorting out. Z-wave don't really know too much about it. Also take a look at PLC bus this is like X10 but much better, there is now a plugin for Homeseer for it. There is also this software for the PLC bus system.
Wouldn't use wireless IP cameras, cabled IP cameras are better. Remember the more wireless stuff you have on a certain frequency the more your bandwidth get reduced, (the slower your network becomes).