128GB of ECC RAM itself is going to be ~800-1000$ as far as I can find some to fit in a Ryzen build. So it won't be trivial to reach the price point for a full build although Ryzen is clearly great value anyway.
True, but only if ECC is necessary. In this case where he is mainly using the server for experiments and to try software, regular ram will do absolutely fine.
Grandparent's point, presumably, was that _buffered_ ECC RAM would not work. Which is definitely correct. I'm not sure if the sticks in question are buffered or not though.
Likely registered, not buffered - FBDIMMs fell out of favor some time ago. Technically still “buffered”, but only the control lines and it maintains electrical compatibility with UDIMMs.