Would you get 12-50mm instead of 9-18mm
I'm tempted about OM-D, but don't know would it make sense to get 12-50mm lens. I already have a wide angle zoom (m.zuiko 9-18mm) that I don't use much. 12-50mm looks much more versatile (and bigger), and I might like power zoom since I like to shoot videos. I probably could exchange 9-18mm to 12-50mm lens if I buy OM-D and 12-50mm together.
If you like videos, I think you have to go with the 12-50. You said you don't use the 9-18 (which is a very nice lens) too much. I believe the 12-50 is perfect for your needs.
(equipment in profile)
The 9-18 is my favorite m43 lens out there. I'm sure that the 12-50 will be a decent kit zoom. But you would have to take the 9-18 from my cold dead hands. You might be better off selling the 9-18 on eBay, and buying the kit.
Why not keeping it and only selling it when you are satisfied with the kit lens?
No I would not, but if I do not already have a kit lens that goes to 14, I would.
I have trouble using 14mm already. 12mm needs quite a bit more of learning I should expect. 9mm would require time that I've not got...
Sounds to me like the 12-50mm is the right way to go for you.
If you plan to by the OM-D, you can get the 12-50mm at a rather attractive price as a kit lens. This would give you power-zoom, which is great for someone who likes video. It will make for a complete weather-sealed package on the OM-D; without it the weather sealing of the OM-D is of little value to you. It is a far more versatile lens than the 9-18mm and should be great for when you are on vacation and cannot be bothered to change lenses all the time.
The only downsides of the 12-50mm that I can see are the size and the fact that you will not have anything wider than 12mm. But from what you write, it sounds like ultra-wide angle is not really your thing. So 12mm should be plenty wide for you - I don't have anything wider than 14mm myself.