Best HTPC setup

As far as your questions:

a) yes
b) no
c) probably....

Do you have cable, directv or over the air? You won't be able to record digital HD channels from your cable or directv coax. You can only get QAM and over the air signals right now. There is a cable card support out that will let you capture cable signals, but you have to buy the whole htpc from a certified dealer. Directv is supposedly coming out with a solution that will let you capture their stuff, but that is not out yet.

You could probably run your mythtv backend server in your office, and then build a nice small linux htpc for your media room as the front end.