there seem to be a reasonable selection of ITX boards that work fine as media directors..
the core however (from what I have seen) should be something more substantial
especially runnung multiple 720p streams!
also you will want to have quite a lot of storage at the core!
I see Brazil is quite strange about its TV standards as you say
maybe satellite capture may be easier ?
make it as powerful and big as is reasonable
if you don't you (like me) will just end up building another core when you run out of slots / drive bays