ok great. 2 questions then..
1. is playing with 1 hero still a fun game?
2. core set box says '2 10-card starting decks'. ie how would you build the starting decks for the 3rd and 4th player?
you can't play 4 players with a single core even if you are only using one hero. There is only two starter decks in the core. You could play 4 player 1 hero apiece with the core + starter though.
One hero game is fun for the first few games but I wouldn't recommend it after that.