How does the lisense work? Is it a one time buy or a monthy thing?

Reading the information about something you want to buy really helps.

http://www.lfs.net/?page=shopinfo

So yes its a one time thing providing you dont want to buy additional content. Upgrades in the simulation are free.

To follow up on that point, once you've bought S2, you have all the content that's available, so you're done buying stuff.