Back in the day, heat was the limiting factor on overclocks and WC gave you significant advantages in this regard. Today the ROI or return on investment is much smaller in that voltage more often than not limits the ultimate OC's a system is capable of. That doesn't mean to say I'd be comfortable running my system 24/7 at those highest overclocks nor be willing to put up with the fan roar.
With a single GPU and CPU, both overclocked ..... you can do very well with air. Of course aesthetically and noise wise, I much prefer water cooling
As far as AIO (All In One) or CLC (Closed Loop Coolers) I see no value to them whatsoever. The reasons to go away from air are to get better thermals and lower noise...... I have yet to see a CLC that does as well as the better air coolers without being so loud that I can't sit in the room with them.
Any of the three air coolers will better serve you than any CLC (in order of preference).
Phanteks PH-TC14-PE
Thermalright Silver Arrow
Noctua DH-14.
As for custom WC loops, you can spend anywhere from $400 to $2500 without really going crazy so it depends on what you want to accomplish.....
Might wanna peek here:
http://www.tomshardware.com/answers/id-1785226/water-cooling-101-bother.html
The top three coolers above at full load in my systems here are inaudible..... wanna heat CLC, look here
http://martinsliquidlab.org/2013/03/12/swiftech-h220-vs-corsair-h100i-noise-testing/
Wanna see how they do thermally ?