As for parts and brands, for slim rads the Hardwarelabs Nemesis GTS range is superb, and what I run. AquaComputer, EKWB, AlphaCool etca LL make good kit.
No reason at all you can't do a CPU loop first. You could even add in a couple of QDCs and just plug in the GPU block when you're ready.
I do love the name of the Hardwarelabs GTS. Nemesis. That was the reason I picked it over the EK slim as I've read EK slims aren't good performers. Just a shame I couldn't make use of the XE in my build.