[Lawura]

Buiding UI is a huge task and even if such tool as you describe excisted, I doubt it would be very helpfull at bottom line.

Best I could advice you while building up UI is:
Look for Guides about UI building. I never seen/used one myself but they do exist

There are tips on some wow-related sites about what addons to use, how to configurate them for specific classes. As example, tankspot.

The visability of your UI foremost depends on your game style. Even after you HAVE actually composed a layout, its without a question, that you will have to play game to make sure the layout works fine in the action. If you understand what I mean. Most likelly you will move your addons not once after you have drawn your perfect layout. If not, you are either a horrible player I would never team with or your a genius, I would worship =P

And to end my post:
There was somewhere I read about adding addons to your UI and choosing their layout. One thing at a time. Play with it, move it, add another thing, All over again. Good luck =)

Hope I did not traced too far away from your question.