The D-1529 can be TDP limited to 20W (at a whopping 1.3GHz), but if you goal is idle power consumption under 20w you are better off with one of the 35w TDP D-1602 since it is only a dual core and has the lowest standby consumption. The D-1518 is also a popular choice, and basically the same as the D-1529 but with a 35W tdp limit so it can turbo to 2.2GHz.
note - the xeon-d series is not socketed, so the motherboard price includes the CPU. Make sure to get board that takes full size DIMMS since there is not a lot of ECC laptop memory on the market.
I'm aware of the Xeon D line, but sadly that was also squarely aimed at the enterprise albeit maybe a smaller scale enterprise. It's not something you can easily go out and buy any more, and when you do it's both very expensive, still PC sized, and not any more power efficient at idle or configured TDP when compared to a new Intel workstation or new AMD setup.
EBay offers a number of Xeon D boards at reasonable prices, many with extensive passive cooling. They have a lot of life left in them, to my mind, with good enough power efficiency for a box with normally very low CPU load. No moving parts, nothing to break. Yes, they are not speed demons, but low TDP presumes that anyway. ECC RAM support is there, though.
The D-1529 can be TDP limited to 20W (at a whopping 1.3GHz), but if you goal is idle power consumption under 20w you are better off with one of the 35w TDP D-1602 since it is only a dual core and has the lowest standby consumption. The D-1518 is also a popular choice, and basically the same as the D-1529 but with a 35W tdp limit so it can turbo to 2.2GHz.
You can get a barebones motherboard (even mini-ITX is available for making small machines) for about $500 or a complete computer for a bit under $1000 ex https://www.newegg.com/supermicro-sys-e300-8d-intel-xeon-pro...
note - the xeon-d series is not socketed, so the motherboard price includes the CPU. Make sure to get board that takes full size DIMMS since there is not a lot of ECC laptop memory on the market.