Great to hear of your interest in Lumion.
If you have not seen these then please refer to:
1. LUMION 4: Minimum hardware requirements
2. Lumion Web Site: Specifications
3. Passmark: Videocard Benchmarks
A Quadro K2100M has a Passmark of only 1390, so it's below minimum specifications for a 'simple' scene. Quadros are designed primarily for CAD based applications, Lumion works best with general GTX/Radeon cards. However, Quadro's are used and work OK, just not to the same level of performance and with a much higher price.
Again, the Quadro K2000 is 1671, just within minimum. The K4000 is 2826. There is not much scope there if the nature of your client work requires more complex scenes and animation.
Refer also the Benchmark
topic graph showing user submitted results. Those Quadros are down the lower end of the chart. We do have some users with K6000's but of course they are top of range and expensive.
It will depend on the nature of the client projects and complexity of scenes, demands for using effects, and detail in the scenes. The intended result is of course Photos and/or video animation. So even though there's great improvements in the upcoming version 5 render speeds, the ability of the hardware to render a scene in a reasonable timeframe is time critical. It would not be good to have a final render started for overnight, only to find that it is only 60% finished the following day, and that it was directly due to limitations of the hardware. That is, allow for scope, and go for the highest level of hardware possible.
That being said, Lumion offers workflow and scene management practices, that help customers with all sorts of hardware get the job done.
If mobility is essential and overriding, then M series cards are required. M cards because of power consumption are a lot less powerful than desktop counterpart. Generally you need to look at top level M cards such as 780M, 970M or 980M. For Quadro M's, even the K5100M has a Passmark of only 3160.
If CAD is going to be a main requirement, then of course, there is less demand for GTX/Radeon based cards and more of a demand for Quadro.
Let's hope your office will find suitable hardware for their budgets and project demands. What ever the decision, hopefully you will be able to proceed with Lumion.