I think it will be difficult or impossible to source materials within that price point new on a DDR5 platform, so you’re likely going to have to source used for some/all components. 64GB of ECC DDR5 alone can be over $350.
If you really need ECC, economy of value lies with DDR4. Specifically I chose Registered ECC DDR4 because the used market actually exists for it. I got 256GB of RDIMM DDR4 3200 for $381 shipped. If you’re curious about what my specs are, my post on that is here: FrenziedManbeast's HomeLab
If you’re a little flexible on price but insist on new, check out this platform and see if it has enough Lanes and I/O for your needs: https://www.newegg.com/asrock-rack-b650d4u-supports-amd-ryzen-7000-series-processors/p/N82E16813140100