I assume you're in Europe so a pre-purchase inspection (PPI) by one of our gurus is out of the question. You should look very carefully for any signs of damage repair.
The Heritage was a special paint option - it was technically a stripe delete + Heritage paint. All other options were available - BBS wheels, radio and painted calipers. It seems that most Heritages had all three but there was at least one non-radio upgrade I heard about.
The only optional item that the Heritage models came with that no other Ford GTs came with is a large cardboard box with 4 sheets of numbers on them for application onto the roundels. The box is 8"x8"x37" and includes instructions for applying and a squeege.
I have that box! Never realized it was an option though