You can't just download UNIX. UNIX is currently owned by the SCO group (which is a joke) and you have to get it from them. Linux and BSD are as close as you're gonna get. They are both clones of UNIX, in that they don't use any of the UNIX code, but feel a lot like UNIX.
Actually, my father teaches UNIX classes, but he cheats; he just teaches SuSE 9.1 Personal and Enterprise Server.
BSD is a more UNIX-like operating system, but Linux is just plain easier to use.
If you do decide to download a UNIX-like Linux, try Slackware. They try to make it as UNIXish as possible.
250-300MB download size---I don't think so...not with a modern release of any distro, except maybe **** Small Linux, which won't do jack for you.
UNIX reqd... 
Linear Mode
