You don't need a terribly fast processor - the development machine is a 233MHz iMac - but it makes tournaments go faster.
The current version is known to run on Mac OS 8.6 through X. Earlier versions may work too. It requires a PowerPC Mac, so really old machines and 68k emulators won't work.
The Windows version runs on Windows XP, Vista, and 7. It is not compatible with Windows 2000, and probably not with earlier versions either. (Compatibility with Windows 8 not yet confirmed, but it should work.)
Linux will run the Windows binaries using the Wine compatibility layer. The native Linux version is headless only and fairly out of date.
Yes, but give credit. If you borrow a type, put an appropriate #author line in the type. If you've only slightly modified a side, leave the original author's name on it and make it clear what you did. (Usually you put a list of changes at the beginning of the side or type, with your name and date next to each one.)