I think Slackware or Debian would be a good choice, personally. I'd stay away from Fedora for the moment, since releases become unsupported so quickly, and if you don't have regular console access to the machine, upgrading is a major pain (been there, done that). A BSD would be OK, but don't expect it to look like OS X (which is a Unix that has been very heavily tweaked by Apple). In general, any distro will be fine if you just install the packages you need. Apt-get will work with RPMs, so that shouldn't be a concern.
Also, a word of warning -- simply switching to Linux will not make you secure. If you're running a server you need to take responsibility for applying security fixes and doing other preventative maintainence to make the machine as secure as possible. Linux and its applications have their share of security holes. I think they're generally patched more swiftly than MS security holes, but you still need to make sure that those updates get applied.