expiration wala, unless stipulated like free to play for a period only. as for OS, syempre kung windows lang ang supported hindi tatakbo sa ibang OS (or kung tatakbo man there might be issues). as for separate download, pwedeng hindi. since once a game is added to your steam library andun na yun kahit pa sa ibang computer mo open ang steam account mo. you just have to manually specify its location in your HDD para hindi na magdownload