Well, your typical AIO is a copper cold plate and an aluminum radiator. This works well enough, but typically it is a bad idea to mix metals in a solution. One will start to leach ions from the other. They put in anti-corrosives, anti-freeze(for shipping in cold weather), and biocidal agents to prevent bacteria and algae growth. Pretty sure all of Corsair's offerings fall under this.
The more expensive models will use Copper and Brass radiators (copper radiator, brass fittings/joints) Copper is a more effective radiator material, but is more expensive. An example would be Alphacool's Eisbear solutions.
A lot of the higher priced AIOs offer RGB lighting, displays (temperature, animated logos, etc), and fan controllers. Depends on if you want to prioritize cooling or appearance.
And finally you have all the custom side of things. That is getting quite a bit more expensive, and isn't what most people need. I suppose what I have could be considered semi-custom, since I use Rad/Res/Pump combo. Typically the pump and reservoir would be a unit. Alphacool, EK, and others make expandable water cooling loops, pre-filled GPU blocks that can be plugged into their CPU coolers with quick disconnects.
Hardline tubing probably close to the hardest thing you can do.
H115i is a fine choice, Corsair stock fans are a little loud when maxed out, but you shouldn't run them at max.