Custom watercooling builds nearly always need to be planned from the very start, not as an afterthought.
Sounds like this is an afterthought with emphasis of 'look I watercooled my PC'. But, fair enough.
Step 1: what CPU and GPU will be used? What will overall TDP be for these components?
Step 2: is GPU standard PCB layout with a) block options, or b)was the 'fancy OC extreme turbo super number 1' model selected because of marketing? Hopefully a) and not b). If b), good luck finding blocks, there might be one, if you are lucky.
Step 3: what case will accommodate enough radiator space to dissipate TDP from step 1? For relatively standard CPU+GPU at stock speeds, a 360 is pretty much considered common as a starting point, or a very thick 240 with very high performance fans. If overclocking, 360 is the smallest you can consider. This means it does not have to be a 'single 360' but can be a '120mm + another 240mm'. Similarly, 2x 240's can also be a 120mm + 360mm. It doesn't have to be 'contiguous radiator space'....just the total surface volume.
Step 4: tubing size, hardline or softline tubing? Barbs or compression fittings (also depends on tubing used).
Step 5: pump + reservoir considerations. D5's are great, but they are large and often take up a lot of space. Also, they are great for flow rate, but aren't as good in restrictive loops like DDC's are. Either option would be good choice unless the loop plan is going to be very complex and restrictive...then consider dual pumps or a better layout.
I'm assuming the watercooling sticky has not been read through thoroughly based on the questions being asked.