There are about 15-20 transitions in iMovie and yes, you can buy plug-ins to give you more. The transitions can be speeded up/slowed down and have their direction changed for the most part. I'm not a massive transition fan - simple cross-fades do it for me most of the time although the 'ripple' worked well on a few beach trips.
The only motion effect for pictures though is a simple Ken Burns effect - panning and zooming in/out
And yes, you can record a voiceover onto the other audio track if you want to.
However, for $99, FCE is a steal so you might want to go for it anyhow - just in case you feel more adventurous in the future.