You're correct, modeling is single threaded but typically for architecture the polygon count isn't so high that you would sacrifice render performance. If you go for faster cores you will get better modeling performance but do your models have that many polygons that this is a concern? I've never ran into anyone who would chose faster cores rather than more cores for rendering. Everyone is different though though, if you don't care about waiting for renders then by all means go for faster cores.
As for the GPU, if you're only using Vray with Sketchup then I really wouldn't worry about a decent GPU because Vray will only use your CPU for rendering. If your firm is even the slightest bit interested in VRay RT or any software like Unreal Engine, Lumion, Substance Painter, Substance Designer, etc. then get yourselves a Nvidia 1070 or 1080.
Stay away from Fire Pro's and Quadro's, they're very expensive and typically used in industries that do massive physics simulations with programs like Solidworks. It used to be that CAD programs would benefit from a workstation card, but gaming cards are so powerful now that you would never notice a difference. Put the money you would've spent on that M4000 and put it towards a better processor(S).