Ideally, one ressurrector as well.
The quest will definitely not be easy, but I wouldn't send all the strongest heroes to the same place. Even if the main group can handle it without them, it would be a waste to fail the quest, for example...
Added 20 minutes later
A couple more points about the quests in Heckel. First, the snakes are very dangerous, with good stats and immunity to mind control. Second, most of the Heckelian non-mages will likely have a passive ability related to poison. That is, stacking (and quite large) penalties to accuracy and evasion. And we don't have a universal cure at the moment. Only Delia and Katavasia can remove negative effects. Moreover, they can only do this a maximum of nine times (unless there is some way to replenish mana) and only if they are not casting anything else (although the latter also includes resurrection healing).