Keep my 20mm f1.7 with this gear...?

Ummm ... no.

Most videocameras today can not keep up with a m4/3 body -- and definitely not a EM5 or GH2. I recently sold off my prosumer video gear and moved exclusively to m4/3 with no regrets. Better lens, better resolution, steady cam IBIS on the EM5. You need to spend significantly more more to get a better image.

And you'll be happy with the 12mm and 20mm. The 20mm is not perfect for video, but as long as you lock focus before shooting, it can be a great tool. I love look and use it often for video.