Bipod

I second the suggestion to buy only the filters that you think
you will use.. You can always buy more later.

Graduated ND filters have to slide up and down to line up with the horizon. This
means you need rectangular filter and a filter holder. The two best known systems are
Cokin and Lee -- hundreds of filters are available for each. Lee is more expensive
than Cokin and slightly higher quality. But I've had my Cokin holder for decades
and it still works fine.

Filter holder advantages:
* Filters can be slid sideways
* Adpater rings support different lens filter ring sizes
* Can hold multiple filters

Disadvantages:
* It takes time to screw the holder to the lens and insert the
filter into the holder
* Holder is bulky tp store and sticks out from the lens
* Filters are made of "resin" (euphemism for plastic) and scatch easily
* Filters are not coated
* Filters (in their plastic boxes) are bulky to store
* Lee and Cokin are incompatable (they do not accept the same size filters)

This isn't as bad as it sounds: scatches can be polished out of many (but not
all) filters (e.g. with Maguires "Mirrorglaze" plasic polish).

Both brands are availalbe in two different sizes (Cokin used to have a smaller 'A' size,
but it's been discontinued). With any filter holder, it's important to get one that
is large enough cover the largest diameter lens you own or plan to own---without
showing up in the frame. Both Cokin and Lee made adapter rings for just about
every lens ring, but if the lens is bigger than the holder, you're out of luck.

I use a Cokin 2-stop, gradual graduated ND (Cokin 'P' size: 84 mm). Let's take
that one a time:

* 2-stop strength. Graduated ND filters are avialable in three strengths: 1, 2, or 3 stops.
1-stop is rarely strong enough for a bright sky, and 3-stops can darken it too much,
which looks unnatural. But of course, it depends on what you are shooting: sometimes
even 3-stops isn't enough. Generally though, too little of an effect is better than too much.
The filters (in their little plastic boxes) are bulky, so you don't want

* Gradual. Graduated NDs come in two-types: gradual transition or
sharp transition.Gradual is better for lanscapes (where the horizon may
not be perfectly flat) and sharp is better for seascapes. Unless you shoot
the ocean a lot, buy the gradual.

* 84 mm is larger than the largest lens diameter I own.

Because the filter holder is so bulky, I prefer round, screw-on filters
most of the time.

I use three screw-on ND filters: 1, 2, and 3 stops. All 55 mm.
55 mm filters work for all the lenses I use them with (55 mm, 52 mm and 49 mm).
My larger diameter lenses are telephotos, and I can't imagine why I would put an
ND filter on a tele: it's already dim.

Don't pay a lot for a great multi-coating---while it might reduce visible flare,
a lens hood works even better. And flare you can't see doesn't matter on an
ND filter, since the purpose of the filter is to block some light. This is one
case where coating isn't that important.

An ND filter has at least two uses: (1) To permit longer shutter speeds in order
to show motion blur (water falls, streams), without having to resort to f/22 or
that's still too much light, or on auto-exposure cameras. (2) To allow older cameras
whose top shutter speed is only 500 or 1000 to expose correctly in very bright
conditions. The 2-stop strength works most of the time, so I'd start with that one.

Half strengths are also available: 1, 1.5, 2, 2.5 and 3 stops.

Hope this info is helpful.