Some explanations:
As you know, even liquid cooling needs fans. The main difference between air and liquid cooling is that the latter sends the hot stuff to another location to be cooled. However, the air used for cooling is still as warm as the inside of your case. If you want cooler than the surrounding air, you'd need a compressor like the one in a refrigerator, or a temperature exchange system like in an air conditioner. The best air coolers are as efficient as liquid coolers and they have less components to potentially fail. And they can be even quieter!
This explains the difference between Z170 and Z270:
https://www.pugetsystems.com/labs/articles/Z270-vs-Z170-What-is-the-Difference-877/. As you can read in the article, there's other letters involved, too. However, since you have chosen an overclockable processor, you'd obviously want a motherboard that supports overclocking. The Z270 is newer and as you can see it has more pci-e lanes. As it reads, that's no big deal. The new SSD disks and connectors are agreeably much faster than the previous ones, but you can't see the difference in daily use.
For RAM, no matter how much you get it, get it in pairs. 2x8 GB for starters, or 2x16 if you so wish, leaving two slots empty for future expansion. That's because of Dual Channel technology: One RAM stick can be written onto while the other is being read.
Yet one thing about SSD's: If they break there's very little to do to get the data back. Spinners can often be read and saved using another computer and the platters can even be transferred to a working drive if the motor stops spinning. Your habit of using an external storage is good practice and much cheaper than any data recovery operations.
