You are right, using the NosOpen data and running a server on them should be legal. The problem is a client as you have said, all client content is under copyright.
Getting a client data is impossible i think, because its specially created and unique for the publisher that is publishing the game, they will not give you rights or maybe sell it for millions.
The best way should be run the whole server and do not provide the whole client, just launcher that will modify some data in client or something like this. So you basically tell the user to insert that launcher to NosTale game folder i think.
You do not provide the client in this case and no one can know from which client is the user connecting so this should be okay.
I think you should be safe if you are running a non-profit server, everything is usually ok until you start charging money for something that is not yours. If you have a non-commercial server they will tell you just to shut it down in the worst case, if you start taking some money or start profiting by in-game purchases, then it could be really worse.