They have no way to tell where you downloaded 1.3 final from, I would disagree on the nullified vs the 1.3 download.
It's not like they can come to your site with the 1.3 and say "You downloaded this board from (site here) on (date) , it's illegal, take it down!" they wont have a clue as long as you never say where you got it. It could have been on your pc forever for all they know.
The nullified one, you never know... it's nullified but no one is really sure, if you do get reported to invision if they have a way to track down people. The only way you'd get in trouble I suppose is if some disgruntled user who suspects you for whatever reason, or if you told someone and they backstab you.
Still 1.3 original is safer to me than using a nulled board.