why not buy a bmmcc + monitor? way better build quality, way better battery life, 60fps, slightly better image quality(not much).
emmm...rolling shutter on bmpcc is almost twice as fast comparing to a7s, calling black magic color terrible is kinda stupid, a7s can't touch it, and a7s it breaks in post way easier than bmpcc raw, moire yes, it rarely creeps in, but when it does, my god it's terrible(olpf is a must), thou as a black magic user, for a simple work imho a7s is still better, because it is way more versatile camera.
the only difference will be that you can't export back to prores on pc(well you can, but not as easily as with mac), but importing prores, and working with it is the same as in mac.
for me, my tablets, phones and laptops are apple's, but for my main computer i use pc.
that's because it's the same 4k sensor, its not a 4.6k sensor!
its 5k, not 3k...3k will give you standart 4k ursa mini, witch uses old bmp4k sensor, that has 12 or so stops of dynamic range (i would say 11 usable). For 5k you will get 4.6k ursa mini, that has new sensor with high dynamic range.