1) Use a Get Nearest node for both - for getting the furthest, enable Inverse Order.
2) Depends on what you want to check for. Generally, the Check Status node is your go-to node for any status conditions. Additionally, there are the Get Status Value, Get Attack Attribute and Get Defence Attribute nodes to get e.g. the combatant with the lowest/most HP or the the lowest/most resistance to fire attack attribute attacks.
3) Depending on the combatant's AI setup, e.g. using the last target, the nearest target or a random target.
4) No, as this all depends on what you want to achieve :)
The Found Targets setting handles how already found targets from previous steps are treated - e.g. Check is used to further filter the already found targets, Clear if you want to start the target selection again.