My suggestion mirrors the two above: start with screw-on CPLs, or at least an 82mm with 82mm->77mm step-down rings. The screw-on filters are, for whatever reason, IMHO a lot easier to keep in good condition. The square filter system I chose for my long-gone Minolta kit was scratched in no time so it became quite useless.

I don't have any sort of GND filter currently, so it's hard for me to say that it's not very useful, but I honestly don't recall a time when I wished I had one. I'd probably prefer my CPLs to be screw-on so they're easier to rotate for alignment/tuning.

I'd also suggest that you start with a CPL and see how often you use it. That's most likely the filter you'd use most often, so if it isn't getting much use, anything else would get even less use.