One thing about using used [,old] and free hardware is that it has power
efficiency problems.

Newer hardware tends to be more power efficient (if you choose wisely).
This means that newer hardware can "pay" for itself via energy savings
alone. Less power consumed, less heat. If you have an abundance of
excess "free" power, that you don't have a better use for, then perhaps
old hardware might be better than new hardware from an environmental
perspective.

A reasonably good example of new vs old is the incandescent light globe
against an LED. You could wait until the old one blows or replace it
and start saving energy straight away.

We have reduce, re-use and recycle; these are all important but
sometimes replace is more important for all sorts of reasons.

Cheers
A.