As long as you plan on sticking with an APS-C body, an 82mm variable ND would work. The IQ takes a slight hit relative to a high-quality standard ND, but a good quality vari-ND will be fine.
However, if you plan to go FF, you might want a couple of standard ND filters instead. Personally, I find that a 3-stop and a 10-stop meet my needs; I have 72mm and 77mm 3-stop and 77mm and 82mm 10-stop filters, all B+W. The 10-stop adds some warmth, which can be corrected by adjusting the WB in post.
The reason I suggest avoiding the vari-ND if you go FF is uneven polarization. At angles of view wider than 24mm (on FF; wider than 15mm on APS-C), a polarizing filter cannot evenly polarize the entire field - skies are banded dark/light, for example. A vari-ND is a CPL stacked on a linear polarizer - that uneven polarization translates to a 'Maltese cross' artifact, worse at wider angles and greater polarization (darker setting on the vari-ND).