The main issue with used server hardware that is a couple generations old is that they use a lot of power. E.g. it's no problem to buy dual or quad port Intel server adapters for like 20 Euros, but these draw 6 / 10 W, while newer ones (e.g. those based on the i210-series) draw much less than 1 W/port.

Similarly older storage controllers require more power even when idle, older CPUs and mainboards draw more power when idle. Fan control in server boards has improved somewhat, and these can draw a lot of power (....just ask any Sun Fire server....) as well.