Is the lens going to be for landscape? or general low-light shooting? why such a wide angle for first prime?
I have a 28mm f1.8 prime for my 5D2 and when using it I never feel like I I would benefit from a prime at that focal range with IS to make things easier.
Nothing that a small dose of ISO upage can't sort out.
That is unless you are shooting in extreme conditions where ISO bumpage won't help that much.
It'll be for low light shooting such as buildings at night, indoor shots, etc, hence the need for width.
Also want to just experience walking around with only a prime lens on the camera.