Most n100 new mobos need ddr5, a few older ones only need ddr4 though. Either is unaffordable right now. Better off going for something second hand which uses ddr3. Will use a bit more power but the break even point vs an n100 + ram will probably be a decade at least.

A pi 5 will cost way more than a second hand mini pc.

Don’t forget the power bill, even if they sit idle most of the time. 12 drives * 5 watts * 24 hours * 365.25 days ~= 525 kwh. That would cost me $157 aud a year for electricity.

If I instead bought a 6TB drive brand new I’d break even after 3 years, assuming the 500GB drives are $0.