Christina, RonW is our ND expert and I'm sure he could give you some good advice.
My primary choice of brands are B+W (brass ring), Hoya (aluminium ring) and Nikon. As far as I know Nikon is not in the ND-filter market.
I think you can still focus using an ND6 filter but perhaps one of our Planteers can confirm that. Probably also dependant on the camera model used.
My ND-filter is a 9-stop ND500 manufactured by Light Craft Workshop.
Perhaps something similar to a Light Craft Workshop Variable ND-filter is the answer. ND values run from 2 to 8. Further higher ranges are available but more costly.
Whatever your choice it may be an idea to purchase a large diameter ND-filter and use step-up adapters for your smaller diameter lenses.
Could save you a few bucks!
Hope this helps a bit.
Equipment: D810 | D300 | D50 | B700 | P5000 and some lenses.
Software: Adobe Photoshop Elements 15 + NIK Software Suite
My motto: To learn more today, than I knew yesterday!