if you have spent the money for a noctilux, you might as well go with a high end filter, like the B+W, which would probably cost about 40 to 50 bucks.
another thing to consider, depending on whether you shoot black and white or color, and how many stops you need to knock off, would be to buy colored filters, which knock off stops as well as give different effects to your shots in black and white.
also, i assume you want nd filters to be able to use large apertures in brighter light. have you tried an ISO 25 film? On a bright day, with ISO 25 film, f/1 at 1/1000 would be pretty close.