Detect quest completion

Hi, my friends and I are creating an mmo, so far we have been able to create persistent saving server (MySQL + PHP) and have multiplayer set up (All Blueprint based).
My question is how could we listen for completion of quests?
How can we tell that a player has defeated x beasts or collected all of a particular item to give them the reward?
Bonus points if you can help with the idea of scripting more quests into the game without the need of recompiling (my guess is a scripting language like LUA?)
Thanks for your time