Well, it's difficult to recommend video cards for madVR, because optimizations may change what certain cards are capable of, and newer features may require more powerful hardware.
A lot of people using lower-end graphics cards also have lower-end CPUs to go with them, so that may affect the results.
If you only want to view 1080p24 content on a 1080p display at 24Hz, that's actually not very demanding, as you're mainly only performing chroma upscaling and YCbCr → RGB conversion steps.
A lower-end card may be just fine if that's all you're watching.
If you want to upscale 720p60 video to 1080p60, or if you have a 4K display, then things get a lot more complicated/demanding. Same goes for interlaced vs progressive.
The new build of madVR (not yet implemented in MC19) introduces "profiles" so you can use different scaling settings for SD>HD conversion than you are using for 1080p content for example, which is very good for lower-end hardware.
But this build also introduces new feature such as NNEDI3 image doubling/quadrupling, which can significantly improve the quality of low resolution sources - but it requires a lot of performance.
Who knows what the future of madVR will be. It really seems that you are unlikely to ever have too much performance.
It's when you start trying to use the advanced features on passively cooled or low profile graphics cards that you start running into trouble.
But if you customize the settings and are willing to sacrifice image quality, you should at least be able to get madVR running on just about any fairly recent card, even if it's a lower-end part.