You can freeze buttercream, and you can freeze cake, so you could freeze them together. Personally I wouldn't though. I would freeze them separately. Even if you make the buttercream ahead and freeze it, I wouldn't assemble then freeze the cake. Just my personal preference.

There are a few great baking forums and websites that have some really great and specific information about freezing and thawing cakes and decorating them etc and timelines. Have a bit of a google and you will find them easily. HTH!

And good luck with the cakes! (Make sure you post a pic so we can see!!!)