You could install Apple's 'server unix' from the early 90s called A/UX. This is unix that runs on mac hardware (any 68k machine with a math coprocessor should run it fine). I'm just going to get it running on a MacIIci I got used. The difficulty is finding a source for this software.
The the BSD distros are great for the SE/30.Its a nice machine for doing simple serving if you have the ethernet card in it. Otherwise there isn't 'much' use for a unix machine that's not on a network. Each BSD has its own abilities, at its core a more secure machine than Unix using its standard defaults (any unix may be secure if you know what you're doing).
A/UX ran with the ability to have a fairly standard apple GUI and running unix underneath (can you say the pre-OSX OSX?). It allowed to run straight without the AppleGUI as a standard UNIX, but then you gave up the ability to run 32bit apple compliant apps at the same time.
Check out this link for news on m68k linux options as a starting point.
http://mac.linux-m68k.org/
Linux has the advantage of more software that's already compiled for it (in your case though, on the platform you're going to be running you'll likely need to compile the extras you want anyhow).
OpenBSD is purported to be *the most secure* by default if that is important to you. They take their security seriously and as a matter of all things they include in their packages.
Good luck and have fun!
[ 02-27-2002: Message edited by: bluedog ]