I'm only going C0. There's always reruns if that turns out to be true.
Put it this way, is there any content that you can't clear with whatever off-field Hydro characters you currently have (likely Xingqiu and Yelan). The answer is almost certainly "no".
Now if you like Furina and want to use her a lot and want to make her stronger; go for it.
I'd really like to get her weapon just because it's so aesthetic for her, but ain't no way in hell I'm chancing that weapon banner. I'll wait til she hopefully reruns with something better. Ideally the EM sword or Elegy bow.
*edit*
Had to look up her first two constellations since I hadn't looked since they moved the "fun" one to C6 in the beta.
Is this worth potentially 300 rolls or several hundred dollars (if you lose all the 50/50s) to you?
They may have baited me if they left the C6 as the old C2, but just for damage increase of the little creaturas? nah.