just get a quality circular polarizer, it acts as an nd 2 filter too, you get two filters in one. if you need more ND, then get the ND 4, that's enough ND there.
Can also try 2 x pl for a variable ND (one of which has to be a linear pl). Gotta watch out for uneven polarization though, especially for UWA as mentioned by night86mare. Problem is you might not be able to find linear pl locally.
IF you're serious about landscape works... buy a 2 stop and 3 stops GND... Lee Filters 4x6 size (cost quite high for Lee Filters and Singh Ray but there are cheaper brands out there)... and it could be stacked and doubled as a 5 stop ND (don't use the clear side)...
from my own experience... 5 stop is normally more than enough...
