From what you've said, I think the current imac would be more than adequate, but we can only guess when the refresh will come. Will it be all the imacs or just the retina? Who can say?
I think it's reasonable to think something will happen towards the end of the year.
If you can wait a few months, I would. Save the money and let it make a few pennies. If something changes and you need to get one, rest assured you're getting a solid machine.