Downloading "free" game clients from unofficial sources carries risks. Always ensure you are downloading from a reputable community-vetted server and scan all executable files for malware. Be aware that playing on private servers is a violation of Blizzard's End User License Agreement (EULA).
Legion is part of the (you level through it, but endgame is current expansion) or on Classic Era ? Actually Legion is not a permanent Classic server – only WoW Classic (Vanilla), TBC Classic (now merged), WotLK Classic, and Cataclysm Classic are official. Legion is not available as a standalone Classic server from Blizzard. wow legion 735 client download free
Note : This often leads to "invisible" models or long loading screens while the game fetches assets. TBC Classic (now merged)