>>291024521
Index isn't objectively $600 better than a Rift S, so I think it's only worth it if you're going to spend a ton of time in VR. A Rift S is probably your best bet.
Whether or not a used CV1 is worth it depends on how cheap you can get it. It's worth mentioning that you can't really get replacement parts for them now though, so you're out of luck if the cable goes bad.