Question Go With Alder Lake for Cool & Quiet ECC RAM Supporting Processor?

I would say a 9950x with the TDP set to 125 watt would make you happier than that 12600

I have 12600Ks in two different W680 boards. You can set the power limits in the BIOS. Buy whichever Alder Lake is cheapest and set the power limit as you please.

Conversely I also have Ryzen 7900 in a cheap ASRock B650 board. At 88W load it makes a joke of Alder Lake. But Alder Lake can have lower idle. Total cost was less by avoiding the the exorbitant W680 prices. It runs ECC UDIMMs with error checking in Linux and the supported DDR5 speed is higher. I haven't tested Windows. But I understand the uncertainty confuses you and some other people.

The only other advice I can give: pick your memory carefully. W680 boards only officially run 2 DIMM per channel configurations at 4000 (1R) or 3600 (2R). At those speeds it might be worse than DDR4.

One thing to consider, is that if you plan on running Windows 10 like many of us as we can't stand going to Windows 11, know that CPUs with hybrid core designs don't run as well with the Windows 10 task scheduler generally. So if buying Alder lake, you may want to consider a chip without the E cores, or you could disable them if that is your preference.

I see you are looking at the i5 12600, but I believe an i5 12400 might be a good chip for you, as it also has 6 P cores, but should be cheaper than the 12600. Alternatively, you could get an i7 12700 and disable the E cores.

As for ECC support, I don't know a whole lot about it on the latest consumer platforms. I understand it is a mixed bag depending on motherboard. That said, what is your setup now? One thing to consider for ECC support, is to build an older system based on compatible X99 board, Broadwell EP Xeon (available very cheap), and cheap ECC DDR4 RDIMMs. I recently did this with my server, now it has 256GB of ECC RAM on the Asus X99 Deluxe. Though I am using a Haswell EP Xeon.