if anyone is interested here is what I settled on:
- case - CSE-523L-505B
- mainboard - B650D4U
- 96 GB of ECC RAM - 48G ksm56e46bd8km-48hm
- CPU - AMD EPYC 4245P
- mirrored boot SATA SSD - used Intel (possibly 5400 pro)
- HBA - LSI 9500-8e
- Mellanox 25G NIC
plus a 45 bay Supermicro JBOD (it has two of the flat SAS connectors).
I have not installed this yet. I gather going for TrueNas Scale is the better choice these days (vs Core)