The best watercooling avaible?

- Honestly, I couldn't say about Thermaltake. I personally prefer EKWB, Alphacool, and Swiftech for fittings and pumps/reservoirs. As far as tubing goes, I prefer Primochill for soft tubing and Monsoon for PETG tubing (However, I will not be building anymore PETG tube style builds; too much hassle).

- ALL custom loops will need maintenance at some point. If you use just distilled/deionized water with either a kill coil or biocide, then it is recommended to change out the fluid and check components for possible growth and mineral deposits at a minimum of every 6 months. Some will say differently, but that is the most consistent time frame. If you happen to use a nano fluid or another style of coolant, instead of water, then you could believe the claims of the manufacturer or not. Some claim up to two years and some claim that you shouldn't use them unless it is a show build.
Most of the fluids out there don't have a lot of reviews and not many water cool with them or even water cool with a custom loop in general. Makes it hard to determine what is what in this field. However, to make things a tad easier, there are YouTubers and others out there testing and trying out new stuff and combos of products all the time. Jayztwocents is a great YouTuber to watch up on and learn from. He is also very well connected with others that do and don't water cool. Great resource.
If you want to check out premixed or concentrate versions of fluids and don't want to go with distilled water, I recommend Mayhems, EKWB, and Primochill. Phesser(sp?) sucks. ALL red based coolant will stain soft tubing. I.e. Blood red, red, yellow, green, highlighter yellow/green, and so on. UV coolants will fade over time and will need more added or need the tubing to be replaced since it will cloud fast as well. Best solution I have found is go with PETG tubing or go with a non-red, non-UV coolant (PETG doesn't stain).

- Unfortunately, I'm not big into the RGB lighting for cases. I opted for a mid-plate that lights up with solid white LED. FrozenCPU.com, Performance-PCS.com, FrozenQ.com, EKWB.com, Alphacool.com, Bitzpower.com are some good places to start looking for parts and ideas. There are more sites out there, but these are the big ones I look at. For the most part EKWB will be what you will find compatible with the components you have. If not, they do have universal parts that will more than likely fit your situation.

- Custom or unique parts are hard to come by on the market. Most of this you will find in cases or backplates for GPUs or something. Everything else is pretty much all done on your own creativity, unless you find someone to do it for you. Some of the best water cooling cases though will be what is considered "Enthusiast" level. They don't have a lot going on aesthetically, but they're sleek and sharp looking. Some also are able to be fully customized. Check out Caselabs-store.com, Corsair.com, Fractal-design.com, CoolerMaster.com, Silverstonetek.com, or Mountainmods.com (if you really want to build something big or from the ground up).

Hopefully, I was able to answer something about this stuff for you. Hit me up if you have any more questions.