A k chip is not really needed, you may save a few quid on a non k chip.
In the absence of any CAD based user experiences it is difficult to say which is better for you, a FX has relatively good productivity often in between the i5 and i7, so with that the logical and economical solutions will be between the i5 and FX chip, but maybe HTT is prevelent in CAD usage which may benefit you going with a i7. I will also say that Piledriver is out soon so maybe a Bulldozer is not the right AMD option to consider.