Magicka and all of it’s DLCs are 50% off at Green Man Gaming currently. I have about 4 or 5 of the DLCs and they keep the game interesting. Vietnam is fun. I know for sure only the host needs to have the Magicka Vietnam expansion and everyone can just join them to play. Not sure about the other ones.
Magicka uses Steamworks so it will require Steam. If you purchase the DLCs you will get a cd key that you activate on Steam.
EDIT: OH yeah. Make sure to pay through or with paypal to avoid any foreign transaction fees by your Credit Card company if you’re in the US.