Do I need to have final cut pro to use motion?

You don't need Final Cut to use Motion but you can't create transitions/effects for iMovie either. You can use Motion to create to composite effects on/with video clips and export the finished work as clips for use in iMovie. So in that respect, you can use iMovie. Motion should make most of the work convenient. You'll need to make sure that the clips you import and the project length are the same (or shorter if you intend to trim the clip in Motion). For transitions, titles, and effects, you can drag clips into the provided "drop zones" for the project type (if you were using FCPX, you would generally leave these "empty" as those drop zones are filled automatically by what exists in the fcpx timeline.)

You'll have to adapt your workflow modestly, but I think you'll find that Motion will be a big help to you. I know people who edit entire (short) videos in Motion. It's an outstanding deal (just expect a bit of a learning curve - but there's a lot of free tutorial material around on the net.) I personally don't recommend that you use Motion to edit video in exclusively, it isn't designed for that (and typically will not play video with a lot of special effects at full speed...) It is a video *compositing* application; more like a "photoshop" for video.

HTH