Im an Architecture student with a gtx 970. I primarily use rhino 5, revit, the adobe suite, maxwell and vray.
Get a gtx, I have had zero problems with mine. Can Lumion use the gpu fro rendering or is it cpu bound? If its gpu then you should get the nicest gtx you can afford. My current Rhino File is 1.5 gb and viewport performance in shaded and rendered mode is great. If you need further convincing watch this video