Well, those old Dell boxes are exactly what makes a good Linux workhorse. Just old enough to have drivers and just new enough to be useable.
Grab a monitor and keyboard and plug it in. Does it boot? If so yes great Linux box.
If so my personal favourite is
here.
Download the X86 ISO and burn it to disc.
Install takes about 15 minutes per computer and they'll fetch somewhere between 40-60 (ish) working.
Hey put that Mac back together and see if it powers up. Working macs are worth money.
However if none have hard drives ignore everything above. Pull the CPUs out (boardsort will probably have those on the list of buy CPUs). Scrap the rest. Dell tower parts don't hold value well. Like hp it's all proprietary or sub generic.