Well for forest, you should use a a group of lot of trees and under your modeler, group them as one mesh
You can make different group number of trees to put on your terrain and use individual trees also !

This tip is heavily used in some AAA engines, a very good trick.
Caus individual mesh entity indeed costs to diaply and manage by the engine.

Yes for detailled terrain, solid shaders, clouds on terrain etc ... UDK indeed is lot more appropriate.

There is so much RTS style games, i hope you'll have original ideas ? gameplay ?