Some comment to your first question, its a bad idea to make the construction of your game dependent on which editor you like better.

Models and levels are different. For instance, when you make your levels with MED, you can not use lightmaps and you can not easily attach actions to models and can not use the templates, so you must do everything in script or C++. Its much harder to do a level only with MED and the result will be not as good.