We experimented with SCO Unix and QNX, which both support Apache, before settling on FreeBSD as the best environment for this exercise. The whole of FreeBSD is available—free—from http://www.freebsd.org, but sending $69.95 (plus shipping) to Walnut Creek (at http://www.cdrom.com) gets you four CD-ROMs with more software on them than you can shake a stick at, including all the source code, plus a 1750-page manual that should just about get you going. Without Walnut Creek's manual, we think FreeBSD would cost a lot more than $69.95 in spiritual self-improvement.
If you use FreeBSD, you will find (we hope) that it installs from the CD-ROM easily enough, but that it initially lacks several things you will need later. Among these are Perl, Emacs, and some better shell than sh (we like bash and ksh), so it might be sensible to install them straightaway from their lurking places on the CD-ROM.
Linux supports Apache, and most of the standard distributions include it. However, the default position of the Config files may vary from platform to platform, though usually on Linux they are to be found in /etc.