Hi,

Yes you can use MED alone with some parameters to keep in mind.

When creating terrain either use the 'create new terrain' command or bring in a hieght map to generate the terrain.
A hieght map is an image of land contours made in black and white format.
The terrain creator reads the values of the colors and assigns hieght by 'light' value so all dark spots will be lower than all brighter spots.
Result is a terrain file

You can use the same process to create your 'Plane' by converting the resulting terrain to a .mdl where you cam manipulate the vertices however you want.
Caution!
If you manipulate along x, y axis you will no longer be able to reconvert to terrain but thats not a porb for your discription is it.

Then any assets you create in Med to fill-in but you know will not have actions assigned to them, give then the action following to make them static objects (model versions of blocks)

action im_static()
{
my.emask &= ~DYNAMIC;

}

Ensure you skin is perfect before importing as static file or ent_creating.

Fbx causes a whole heap of issues and one of them is 'tearing'where edges seperate during engine build anad results in gaps in the geometry.

HAving gotten all you level elements made, then the character and so on.....
and finally, build in 'Simple Mode' only as nothing within the Wed build process supports the level you just made as Tobias iterated above.