The very first system I ever built (sig rig), I put a custom loop in for CPU+GPU+motherboard. Keep in mind though, I'd been keeping up with the computer industry and doing general research on how to build computers for the 3 years before that. Once I'd gathered enough money, I built this baby and, believe me, it was worth it. All I can tell you is: research research research, and it seems you're doing plenty of that. As long as you know what you're doing, what everything does down to the most minute details, and large amounts of patience, you'll be fine. If you're only going to cool your CPU, go with the kits charlie mentioned, those are top-notch. If you want to add in your GPUs, I'd recommend going custom. It just involves buying the parts individually instead of buying a pack which includes all them.
EK is a well-known watercooling equipment company, they're currently (or at least were) the champions the CPU cooling category with the EK Supreme HF. They have all kinds of blocks from CPUs to GPUs to full-board mobo blocks.
So, once again, do your research, order intelligently, and LEAK TEST. I cannot stress this enough. It is impossible to damage your components with water if there isn't any power going through them. First time I assembled my loop, major leaks left the GPU and left side of the motherboard completely drenched. Left it out to dry for a week, re-assembled more tightly this time, everything was fine.
Good luck, and don't be afraid to ask for any clarifications on anything!