Paul,
Though Robert and I have a common interest in subject matter and shooting with color slide film such as Fujichrome Velvia and Provia 100, here is a case where we have a minor disagreement. I know that Robert has suggested a kit consisting of .3 (1 Stop), .6 (2 Stop) and a .9 (3 Stop) filters, I would disagree with the 1 Stop ND Grad. Personally, I do not find enough of a difference to warrant the use of the filter. At least to some degree this can be dealt with in the darkroom or computer.
From my experience I will commonly select a 2 Stop or a 3 Stop ND Grad filter. I do not normally find a problem with the 3 stop filter (only a photographer may readily realize its use, but if used properly, it normally looks natural). These Grad filters are used primarily for transparency material which has a narrow recording range of 5 stops maximum. I normally expose for 3 1/2 to 4 stop range so that I can get some detail in the highlights and maintain some detail in the shadows.
The filters are usually selected to allow the film to record within the film's latitude. Therefore as an example, if there is a difference of about 4-4.5 stops between the foreground and the brightest areas of the background I would normally select a 2 Stop ND filter. However if the difference is about a 5 to 5.5 Stop difference I would select a 3 Stop ND filter.
As to the selection of the manufacturer of the filters, I would normally stay away from the Cokin Grad filters, they are not known for being very neutral. As to the size of the filters you have the option based upon the filter thread size of your lenses. I know that Robert and many others use the Lee System which I also have. But if your lens filter threads are smaller you can get and I am also trying to standardize to the Cokin P Holder system filter size (this size will also allow the use of the assortment of the special Singh Ray Polarizing filters [including the new Lighter Brighter Warm Polarizer] designed to fit the rear slot of the Cokin P Holder). High Quality Grad Filters in the Cokin P size can be gotten from both Lee and Singh Ray. The Singh Ray filters are a bit more expensive. Both manufacturers also make their Grad Filters to fit the Lee system as well.
Rich