I'm no expert on filters. I will assume you are using a UV filter as a matter of course. I would suggest a decent circular polarizer as well. I know Cokin is a good name and has been around a while. My filters are either Sigma, Hoya or Tiffen. I think quality wise they are all decent and all around the same quality. I see the graduated NDs on Amazon for around $20.

Kort Kramer - Kramer Kreations