Cheapest AM4 option for ECC and GPU passthrough£

Can't help you with the GPU passthrough part. Every recent AM4 motherboards I've dealt with has IOMMU option in the BIOS, so I assume it works, to some degree. But again, I've never tried it, so....

As for Unbuffered ECC memory modules, I always start at crucial.com
Shop RAM -> "Server Memory" and pick the right ones.
VLP ECC UDIMM & ECC UDIMM, and DDR4 for AM4

Looks like they stopped selling slower than 3200 DIMMs, which makes sense.
3200 is the JEDEC spec, so there's no reason to buy anything slower.

I used to use a pair on my 2700X, and to my surprise, the BIOS defaulted to 3200, and it seemed to work fine, but the NB voltage was way too high for my liking, so I underclocked it. The same DIMMs are now running with newer Ryzen (3000/4000/5000 series), so I run them at default 3200MT/s.

I'd make a note of their price, and then search online to see if there are better deals. I've bought direct from CDW, Kingston, Newegg, and even Amazon before. But my current go-to is Crucial, because I know exactly what I'll be getting in the box.