Hi,
Usualy you won't face any problems with processor if you're planning to run ragnarok only, and the bandwith capacity is okay too, except if you're planning to put the website ( control panel - forum ) in the server aswell because it consumes bandwith quickly depending on your visitors and some hosts don't give enough bandwith for that, so it might get your server down, 500MB - 1GB bandwith capacity would be fine.
Ragnarok doesn't need much ram (Memory), i have an average ragnarok server (20~50 population) the map server consumes 300MB, char server 10MB, login server 7MB, so if you even have just 512MB, it will be enough to run a server like mine, but i suggest you get +1GB RAM, so you don't face problems in the future.
HDD, that is the file storage capacity for your server, you don't need much of that.. 10GB - 20GB would be just fine for ragnarok files/database, programs needed..etc, and most hosts give alot of storage, so it is not a thing to care about.
Finaly the Bandwith speed / ping, they are almost the same, most of the hosts provide either 100Mbits/Sec or 1GB/Sec, 100Mbits is very okay for even 500 players online, but if you get alot of players a really good number like +1000 then 100Mbits might not stay enough, then you'll need 1GB/Sec, so it doesn't matter anyways as a start because you won't reach 500 players directly..
About what do you need to worry about if the server gets more players, is the RAM, and Bandwith speed, processor is fine as long as you run ragnarok only.
I guess that's it.
Good luck finding your host.