I respectfully disagree with Puddleduck. Their filters are excellent quality and you get what you pay for.
In answer to the OP's question, I'd look at getting a set. I would imagine a 'hard' set would be best owing to the size of the sensor. I recommend giving Robert White's a call or even Lee Filters themselves as they should be able to give you advice on the which will suit you best.
With the sets, you get three densities: 0.3, 0.6 and 0.9, which equate to 1, 2 and three stops respectively. I tend to find that of the three, the 0.6 gets the most use, so if you wanted to buy a single filter, I would suggest this may be the one to go for.
Hope that helps
Cheers
Mark