Your video card would largely be your biggest consumer of power; however, if you're not doing SLI or Crossfire of high-end cards, you should be fine with 750W. I don't see you going over 50-60% usage in most cases.
Custom water cooling loops are definitely nice in regard to noise and thermals, but suffer a bit from high cost and time. One thing that frustrated me the most is that you usually can't do anything quickly anymore in your system. Swapping a video card goes from being a quick, five minute job to potentially a half hour at best. This isn't a horrible thing, but it can be annoying. Also, cases seem to be designed and maybe some radiators, but pumps have largely been an issue for me. I first used a Corsair 800D, which was fine, but it just didn't have good room for radiators. I swapped to the Corsair 900D, which had tons of radiator space, but it has nowhere to put the pump! (I also built a custom setup for mounting the reservoir that didn't involve drilling into the case.) Thankfully, companies like EKWB have fan-based mounts, which help alleviate that issue.
I'll admit that I was tempted to go with water again, and fortunately, I do have some of the components (pumps, reservoirs, etc.) left over, but I'd still spend a few hundred on blocks and radiators. To note, I also didn't really use quick disconnects in my previous systems. Using those can help alleviate excessive time due to allowing parts to be removed separately without a full loop flush.