When purchasing health at the healing fountain, I'm assuming that when you pay 1 gold, you can heal everyone in the part one HP, right? I can't recall any clarification on this in the rulebook. But if it was only for healing one point on one player, it makes the game unbalanced as more players equals increased difficulty.
You don't purchase health at the healing fountain, you just heal everyone back up to full health.
From the rulebook page 5:
After the encounter phase, all heroes are fully healed
during the clean-up phase.