It's not my language, but let me give you an idea of what you need.
RO itself takes up between 200mb to 300mb of RAM, with 0 people connected. Also factor in how much your server itself will use. Assume 500mb with 0 players, just to be on the safe side.
As far as processor speed and hard drive space, those don't matter so much. Any server should be fast enough to run RO, and it doesn't take that much space overall.
As far as bandwidth,it seems they have 5000 GB as the lowest amount per month, this will be just fine for your server, and as it grows you can simply monitor usage. 5000gb should be able to support a few hundred players who play daily, but I can't give you an exact count.