For 5 computers on a mostly temporary basis you are making it way more complicated than it needs to be. Building 5 computers and placing them in the office where ever is the best answer here. Have a watch of the LTT video they did on remote working, that looks like a good solution when you need 3D graphics, video, CAD etc.
Doing this with a single system to meet your requirements it's mostly impossible, you would have to self build and that just isn't a good idea in a business environment honestly. Vendor servers don't often take more than 4 GPUs, they will only come with and work with Quadros and Teslas due to power cable location on the GPU and servers that can take more than 4 GPUs are much more specialized and expensive.
But even if you build it all yourself because of your requirements you are driven immediately in to a dual socket platform for Intel or single socket for AMD EPYC, either way the CPUs are expensive and you have to use Registered ECC RAM and a lot of it so you're at $1600 minimum for the RAM and up to $3000 ish.
Then you have to contend with Nvidia not allowing Geforce GPUs to be used in virtualization environments, you can get around it but it's another hint that it's not a good idea.
Can you build this for under $18k? Yes. Should you? No.
If you long term want to go down remote working from anywhere any time then you'll want to do an RFP/RFI with some IT firms to setup a VDI environment properly and a proper project from people who are experienced and done it before. To get a good user experience with VDI it's not as simple as hardware, good internet, and picking the right software. You can have all the right ingredients and still end up with a deflated souffle.