You need to have OS X Yosemite 10.10.2 before you can buy iMovie 10.0.7 from the Mac App Store. If you're using an earlier version of OS X Yosemite (10.10.0–10.10.1), the Mac App Store stops responding when you try to install the latest version of iMovie.

To resolve the issue, update to OS X 10.10.2 or later before you buy iMovie.