I'd use caution with the Mi50s. I bought a 16GB one on eBay a while back and it's been completely unusable.

It seems to be a Radeon VII on an Mi50 board, which should technically work. It immediately hangs the first time an OpenCL kernel is run, and doesn't come back up until I reboot. It's possible my issues are due to Mesa or driver config, but I'd strongly recommend buying one to test before going all in.

There are a lot of cheap SXM2 V100s and adapter boards out now, which should perform very well. The adapters unfortunately weren't available when I bought my hardware, or I would have scooped up several.

I've seen the sxm2 (x2) with pci extension cards out on ebay for like $350.

The 32gb v100s with heatsink are like $600 each, so that would be $1500 or so for a one-off 64gb gpu that is less overall performant than a single 3090.