It will be hard on A6 engine.

why not considering Torque or BV engine , both have complete
realtime terrain editors.
Torque basic has not a real shadow system, but lightmapping can do it and mixed with Torque lightening kit (included in V1.5) it will be ok for lot of real time lights on terrain.

BV has any number of texture layers you want to paint on terrain, you can define the scale of texture tilling also.

I think you could find more people on these engines than on A6 that don't have
real time terrain editor...

Just my thought ... if you are serious and will pay for serious art,
perhaps some real game project ?
why not beginning by choosing an engine adapted with tools for terrains ?

It's just to help you on your decisions and clarify your goals