I have B+W Käsemann CPLs, they are excellent. VariND filters can be problematic at wide angles, as mentioned above - a variND is basically stacked polarizers, and the same uneven polarization that results in blue skies banding at wide angles leads to a 'Maltese cross' artifact with variND filters. However, the effect is worse at wider angles and darker settings, so if you won't go wider than 24mm and don't need max darkness, they can be fine. Personally, I use a standard NDs in 3- and 10-stop density, becuase I use the 82mm filters on my 16-35L II and TS-E 24L II, in addition to the 24-70 II.
Getting the 82mm filters and a step-up ring is a good idea from a budget standpoint, just be aware that using the step-up ring often (but not always) precludes use of the dedicated lens hood.