I would recommend kits/combos from the following:
EKWB
Swiftech
Alphacool
XSPC (depending on selection, some older X20 pumps were not very good, but they also do have D5/DDC pump kits)
EK's aluminum kits have versions that also include GPU blocks, but these can only live in an all-aluminium ecosystem, so you would have to always, always, ALWAYS use only aluminum components. Otherwise, brass, copper, nickel, gold and silver can be used together just fine.
We need to know more info - what are you wanting to accomplish? There are so many options to build watercooling loops, that before you get to recommendations, we need to know your plans, budget ($300?), components cooled, case, etc.