Ebay.
Also, i3 doesn’t really use less power. The -T models will use a lot less power. But, you aren’t really going to notice a difference with the i3 models.
TDP is the same between them too.
For reference, I have 3 micros. i5-9500T, i7-6700, and i5-8500t. They all use pretty much the same 8-12watts of idle power.
Also, I generally avoid the pre-6th gen computers. DDR-3, slower, less efficient. i5-6500 is the oldest processors in my lab.
And- right now, 50$ is the going-price for M900s / Optiplexes/etc, with an i5-6500t.
Although, you can get the i3 models for 30$ or so.
I use technitium as the primary server, with a pair of backup servers running bind9.
The backup servers do zone-transfers from the primary.