I need some expert advice, please.

[Major Nikon]

1. Here's a few things you might want to consider

Assuming you'll be using screw on filters, first you want to figure out what size you need. If you have multiple lenses you want to use filters with and those lenses take different sizes, going with the largest size and then using step down adapters can save you a lot of money.

What brand you should be looking for depends on how much you want to spend. I usually settle on either Hoya or Tiffen filters which are generally a decent balance between price and quality. Multi-coated filters are superior due to their coatings which correct for color aberrations, but they can add considerably to the price. For instance, Tiffen's MC grad ND filters cost almost twice as much as their non-MC filters.

One thing that is very handy, especially for filters you will be taking on and off a lot like grad NDs, are the Xume magnetic filter holders. Basically how they work is you buy one lens adapter and then buy a filter adapter for as many filters as you want to use at once. Instead of screwing each filter on, you just pop each one on and off and the magnets hold everything in place. Not only is this more convenient, it lessens the chance of damaging filter threads from cross threading.