Bre of Clan Stoutarm

Bre's last ability will check as your end step starts to see if you gained life this turn. If you didn't, the ability won't trigger at all. Gaining life during your end step won't cause the ability to trigger.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you choose to cast the exiled card, you do so while Bre's last ability is resolving and still on the stack. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.

Since you are using an alternative cost to cast the spell, you can't pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.