I've been using a NZXT Kraken X62 to cool my 5.1 GHz overclock now for over three years. Steve @ GamersNexus has been recommending it for years.
As to whether it is worth it is up to you. Don't expect large gains from a chip that already does 4.5GHz all core boost and is pretty much guaranteed to do 4.7 without messing too much with voltages. Also, depending on your use case, money towards an AIO could be spent better elsewhere. For example, in the context of gaming, a better GPU will net you more gains than a +600mhz CPU overclock.
In my experience with this chip, overclocking success has more to do with the quality of the chip than the quality of the cooling.
Yes, it's worth it to overclock, although you might need to delid to get the best performance from an AIO (or high end air cooler). You should still be able to get all core up to the 4.5-4.7Ghz range. A good chip will hit close to 5Ghz with a delid.
As for the cooler, I'd get at least a 240mm AIO. You might not need to spend as much as you think as the higher end coolers are usually just ones with better fans and MOAR RGB! If you have fans already, you don't have to go crazy. I've used the Corsair H100x ($69AR) and was looking at the cheaper CM 240mm for about the same price. The Krakens are nice, but they are a lot more expensive, and with a 4C/8T chip, might not make that big of a difference. I just went back to my OG Scythe Fuma for my current build, and it's not really a huge difference over a 240mm in most cases.
NZXT Kraken X6 is a pretty decent AIO with the 6th gen Asetek pump. However IMO, unless you really enjoy the overclocking process, you would be better off with a gen 9 or gen 10 processor. They would be faster than 7700K on stock speed and they will run a little cooler while overclocked, as these has a soldered IHS. Yeah you can delid the 7700K but that involves some potential risk.