If you want cheap, but non-performant, there are AMD R-series like found in Dell/Wyse thin clients. They're pretty weak machines though 2-4 cores, but support ECC DDR3 single channel.
Internal ECC doesn't protect the data in transit. It also doesn't have error reporting, so you could be operating with the error correction already operating at maximum capacity without knowing it, leaving no error correction available for unusual events. It's not a substitute for traditional ECC.
DDR5 ECC is an internal mechanism just to make DDR5 viable due to the speed and node size. It doesn’t increase reliability, in enables not having a reduction in reliability over DDR4.
I'm annoyed at the lack of ECC-capable sub-ITX NAS boards. I have a Helios4, but I've no idea what I'll migrate to when that dies.