If you want a simple plug & play kit that gives you everything you need to get a loop going, I highly recommend taking a look at EK's Quantum kits. That's what I used as the base for my current Liquid Loop, I bought a P360 Quantum Kit, added a 480mm x 60mm radiator to it & bought some more fans.
The P360 kit specifically, if you add all the parts it comes with up separately, has about $700 of value in it, and unless they changed the price, it should be between $519-$549:
https://www.ekwb.com/shop/ek-quantum-power-kit-d-rgb-p360 I think other liquid cooling companies sell kits like these too, but the EK one is the one I got & I'm quite happy with it. The quality of parts they use in the kits are great, D5 pump (which is the best you can get), quality fittings, etc...
Oh, make sure you buy extra fittings - most pre-done kits come with enough fittings to do the loop that comes in the kit, it won't come with extra fittings to add your GPU in specifically.
If you do go with EK, I highly recommend their Black Nickel fittings as the separate purchased ones, they have a dark gunmetal look to them and do not show smudges/wear easily at all compared to the normal painted black ones that come with the kit.