Hello there, and welcome to the forums.
Firstly, your background is extremely similar to mine, so the first thing I will say is don't bite off more than you can chew. Trying to do all of this in one go will ruin you. LinuxMCE lends itself very well to "baby steps" i.e. get a core with few TV cards and maybe get one or two MDs going, enjoy that for a while, and then add more stuff later when you have a better idea of what is going on (plus you'll get to enjoy using LinuxMCE in the process).
I'm sure that most people here who know the system inside out would agree that this is the way to do it.... for the likes of you and me, I would have said that it was even more true.
To answer your first question, any half-decent computer from any time over the last several years would be fine. My core was quite out of date when I bought it a few years ago, and it's still going strong. I went on ebuyer.com, and bought the cheapest AMD AM2 mobo I could find, the cheapest dual-core CPU I could find, couple of gigs of RAM, nasty case that cuts my fingers, job done. It's a common misconception that you need something fast and fancy; this is not the case. Just make sure you add a second NIC.
There are several ways to get audio in multiple rooms, but the "traditional" method would be to have an MD in each room with amps and/or speakers connected up to those.
Search for USB-UIRT and zantech emitters re your IR problem.
No idea about Insteon.
Regarding cameras, see first paragraph
Don't forget you can also post DIY questions with regards to your installation in the users section should you need to. e.g plastering tips, fabrication, etc.