From what I understand, you want to build modular pieces that you can re-use, so while you could perhaps lay it all out in say maya then import it, it'd be inefficient, and dealing with things like collisions etc a pain. So with modular assets you'd piece it together in udk, I don't know of a snap to vertice method in udk…
I wouldn't measure out the room like that. I'd say that each wall section I make will be 128/256/512/(16*n) units wide. Then I place them in the level. If it takes 5 sections then its 5 walls long. I don't really focus on the whole. That occurs after placing the elements. The pillars don't "need" to be on the grid for this…