Dumb upgrade question

Ok, this is pretty basic, but my excuse is that I'm new to the world of mac's.

I have an iMac running 10.7.5 and have just started to use Aperture seriously, very impressed as I could never get on with LR. All is well.

Now I also have just acquired a 2010 Mac book pro that had been sitting doing nothing for a while and I've just had the battery replaced by Apple. Once it was working I updated everything the updater told me needed to be updated. All is well

My intention is to load Aperture on the mac book for tethered shooting

However the mac book is running 10.6.8, Aperture needs 10.7.5 (Sorry, no idea what the names of the versions are......... mountain lion, mangy cat or whatever)

So, two beginner questions:

Do I have to purchase 10.7.* for the mac book ?

Would this be a complete installation that blows everything away and gives me a clean installation or if it doesn't, how do I do that?