The minimum I recommended quadro is the old Quadro 4000. It can be used in 2 modes:
+ Virtual Hardware Shared GPU accelerated (which is limited to DirectX 9). This is vSGA and you can share the same GPU among multiple VM, limited by memory. Quadro 4000 has 2Gig and can easily support 8 VMs as long as you don't need anything beyond DirectX 9 (no OpenGL, CUDA in this mode).
+ Virtual Dedicated Graphic. This is vDGA via pci-pass-thru. You have access to every feature of the card, DirectX/OpenGL/CUDA etc. One Card per VM. Quadro 4000 is a single slot card so you can stack 4-5 of those in a box easily.
I think you are basing your suggestion entirely on what VMWare ESXi uses. I never used ESXi, only Xen and QEMU-KVM, but these features aren't really exclusive to it.
If sharing the GPU is "limited to DX9", it probabily means that it provides a virtual GPU that does API Forwarding and thus is limited to the APIs that it has implemented. This is also supported in other Hypervisors that can provide paravirtualized Video Cards with a front end/back end pair, so it can get the job done on the host GPU. VirtualBox does that, and QEMU-KVM has the VirtIO GPU/Virgil3D, but it only works on Linux for OpenGL so far. If they get Windows and DirectX support, it will have feature parity with ESXi vSGA, but without the specific Video Card requeriments.
Passthrough works theorically on anything. nVidia actively wants to play against Passthrough users of their consumer GeForces, but QEMU-VFIO can workaround that (This is why for Xen you could only use GeForce @ Quadros, they weren't aware of how to workaround it).
The thing which I never researched about is how to effectively get video output of a VM with a Passthoughed Video Card with no direct video output. Be it on the same system that host via a virtual bridge or a remote system in the LAN, routing the video somewhere else would take an enormous amount of bandwidth, or performance and quality loss due to have to compress it in real time. Not sure how to deal with that.
You can also use vSGA and vDGA with the GRID (4 for K1, 2 for K2) but there's no point. Please note vGPU is a nVidia technology with ESXi so if don't use ESXi you are limited to the other 2 methods.
nVidia was colaborating in a vGPU interface for VFIO to use their GRIDs. There were patchs for it. Not sure how much it will take to fully implement.
I have 4 quadro cards in my setup; 3 in vDGA and 1 in vSGA. My "back up" has Quadro 2000 in vDGA. I like Quadro 2000 since it doesn't need an extra power connector but at a much reduced performance vs Quadro 4000. Quadro 4000 is around $75-$80. Quadro 2000 is around $50.
I wouldn't feel very happy with a 6 years old piece of Hardware. The Quadro 4000 is GF100 based (GF465/470/480), but with near half of its units disabled, it is actually slower than a GF465.
Besides, it may not have UEFI GOP support in its Option ROM, theorically forcing you to only be able to do Primary VGA Passthrough with it with a BIOS Firmware for the guest VM. There was a tool (updGOP) that could patch old ROMs to include UEFI GOP, but I don't know if it also works in Quadros since I don't recall anyone using it for them.
Other hypervisors are not as picky as ESXi in vDGA (vDGA works with just about any cards in KVM for example). The real advantage of other hypervisors is to use high end consumer GPUs (GTX1080 for example) for games. If you don't game in your VM, previous versions of quadro cards work very well.
The fact that you can throw anything to QEMU-VFIO and expect it to work is what makes it soo ridiculous great.
