To edit terrains, I once used CameraPick() along with TFormPoint() to find the x & z coordinates on the terrain, then you can edit its height from there. For your resource problem, maybe you can split up the terrain into smaller sections, and the farther you are from each section the less detail there is to it. I know that detail control is what Blitz does already, but more importantly, splitting up terrains means areas behind the camera aren't rebuilt every frame.