I run various Debian and Slackware distros on way old hardware (ubuntu, lunubut, linux mint, server variants of those.. slackware starting back in 96.. redhat back around the same time as slackware but never really cared for it)..
Youd be amazed what you can run it on. If the unit is really, really old run a more compact version of linux (of which theres a bunch based typically off the larger distros). For example for acting as more of a vpn server base (but obviously can expand its roll) there pfsense and others... theres also damn small linux which is a tiny base requiring little resources although im not sure on its current status.
To give you an idea, I have a spare junk acer aspire with a 1.8ghz pentium d with 2 gigs of ram currently running dhcpd, plex media server and also twonky, openvpn, tightvnc/xrpd, nagios and I cant remember what all else right now while I build a new machine thats going to run ESXi so I can split stuff up a bit.. but even with all this I can rip one of my dvds to the hard drive for plex to use while also streaming to a roku (transcodes down to a 3Mbps stream since its just for kids movies) and also streams to one of the dish joeys.
So yeah, you can typically run linux just fine (minus maybe any fancy desktop manager.. lxde is a good choice if you require one)
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