I don't (specially DDR3-era stuff) because old server hardware is way more expensive, won’t be of any particular advantage and older hardware, compared to new stuff, will use a LOT of power.

Instead use regular desktop/laptop machines as they’ll probably be more than enough for homelabs. You can a good 9-10th gen Intel CPU and motherboard that is perfect to run servers (very high performance) but that people don’t want because they aren’t good to play the latest games. Modern hardware = less power consumption, cheaper, more performance.

If you go really low end, let's say i5-6500, this will probably cost around 80€ second hand with RAM. You can use https://www.cpubenchmark.net/compare/ to compare CPUs the server hardware you can get with modern hardware if you're interested.

Most DDR3-era server hardware comes with RAID controllers/cards and other things that nobody uses anymore, people have moved on the software RAID be it BRTFS or ZFS and you will want to do the same. Servers make a lot of noise - impractical for a home - and a CPU from that era will be around 150-200W, you can get a recent i5 with more performance that runs around 50W.

Another thing to consider: you're trying to build a NAS get a basic motherboard with 4 SATA ports and then add a PCI to 5 SATA port card and it will be much cheaper than whatever server hardware. BTRFS as your filesystem and its RAID if needed. Now you may be thinking something like “I want a faster CPU in order to have fast SMB”, just don’t - your gigabit network will saturate before an i5-6500 or any mechanical drive does and when this happens you’ll be at something like 10-20% CPU usage. Just don’t waste your money.