I’m taking about machines with maybe 256MB-512MB of memory and a CPU around the 600MHz mark. My new thing in these situations is to deploy the latest Debian in a slimmed down form. More here
The advantage is that you end up with a completely up to date Linux distribution and it’s still standard Debian underneath it all. As it’s a real Debian install, it gives you security along with something that can be upgraded with standard software.