1: Either one will do. You really only need the serial key for the module.
2: The modules don't actually come with any extra ME units, those come with DCS World which is free.
3:I'm not sure how much they go off but I doubt it would ever go 75% off. Maybe 50% at most.
4: I don't think the ME limits the number of units in a mission but your PC hardware might.
5: Yes, there are a lot of active servers.
P.S. why did you write this with big bold letters?