As you mentioned it first, I will make comment on the aspect of your clients requirement re "realistic photo render".
Lumion is not a raytrace render so does not pretend to provide photo realistic renders. It can however get close sometimes, and in some scenes do at least as well if not better.
What it does provide is a rapid workflow, rapid rendering, good to excellent quality output and easy to use. But because it can do animations as well often in the same timeframe as images, io has a big advantage.
It depends what your clients are really after. And, perhaps your builder and real estate customers would not be able to know or tell the difference or that they were not being presented the 'photo realistic' option.
Recommended specifications do not require a GTX680, you can use lessor hardware. I myself use a GTX660 and previously used a GTX560.
1. LUMION 3: Minimum hardware requirements
2. Lumion Web Site: Specifications
3. Passmark: Videocard Benchmarks
What hardware is required will however depend on the type and nature of the scenes you want to work with. That is how complex, detailed, number of objects, etc and how much you can gain from investing in more mostly hardware but get the return from faster scene builds and renders.
Quality of image does not depend on hardware. Hardware affects how long it takes to render.
Lumion 'performs' (almost) the same on all standard graphics (and some professional graphics re Quadro's) hardware because it is doing the same tasks. Better hardware however will either let you increase the scale of the scene because the GPU has more VRAM to hold data. Or, give better frame rates when building the scene, or render images and animations faster because the GPU can process things faster.
The type of scene - whether interior or exterior does not affect the type of hardware required. Lumion tends to be used more for exteriors, however that is changing as well.
We are not able to recommend hardware as such. What I can say is consider the type of work done or required or aimed for, review the guides on hardware as above, do a search for further topics in the forum to get a more in-depth feel for what might be needed, and make a choice first to get Lumion (as you are doing
), then what hardware gives a best fit to all those and your budgets.
A more direct response of course would be; buy the best system you can get your hands on.
Currently that would be something like a Titan or GTX780, although the new Radeon (only top product) is also supposed to do as well as the Titan, along with a high performance CPU, motherboard and other supporting hardware. As you are not involved in any design/CAD work then you do not need to consider where Quadro's fit in with Lumion.