The 100mm filter holder:
The adapter ring, 82mm and 77mm were for my previous UWAs, I am currently using the 72mm for the FE 16-35:
Combine both:
You can try Photosphere as they carry a wide range of filter accessories:
http://photospheresg.com/collections/photosphere-haida-dofilter-100mm-slot-in-filter-system
The set I am using was acquired few years ago from Orient Photo at Sim Lim Square. The beauty of the slot in system is, even if you change lenses, there is no need to overhaul the entire filter system. You only need to get the appropriate adapter ring according to the filter diameter and you are good to go. This set of mine had seen 2 UWAs and currently is on the 3rd one which is the FE 16-35. I recommend you get the 100mm instead of the 83mm size to avoid vignetting problem especially when shooting at 16mm.
For a start, since you are using the A7 series, it means you have a wide latitude of dynamic range to play with. For me, I rely 100% on ND filters only as the DR of the A7R is so wide that I have little to no problem when recovering details during post processing. If the DR of the scene is too wide to cover in a single frame, then I will just exposure blend a couple of frames to reign in the DR.
The ND filters I have are the Haida 3 and 10 stops plus the Lee Big Stopper. Sometimes I do wish I have a 6 stops ND to drag the shutter speed a little more but since the performance of the lens and camera is good, I normally will just stop down and use the aperture to control the shutter speed. I use the Haidas a lot as I prefer the neutral to slightly warm-ish white balance over the cooler Lee Big Stopper. I cannot speak for the CPL as I never use one before.
Maybe you can think about what and how you shoot, then you will have a clearer picture on what to buy. I used to have the HiTech GND kit set which consist of 1, 2 and 3 stops soft grads. The 2 and 3 stops grads saw the most usage but I stopped using them due to the purple colour cast which resulted when stacking filters.