Hearthpwn

Hey, 
I am mainly focused on achieving task 7 for a mercenary and want to grow my collection at all before focus on a single one. But I started to think maybe if I complete task 15 or later the rewards would be better. I read somewhere that task 18 is the last one and gives a free pack. 
Can someone share what are the rewards for the latest tasks - for example 14, 15,16,17,18. And what happens after task 18, you can no longer get a stranger for this merc or you simply get few coins.