As big as your case will fit. The Corsair H80i if you only have 1 fan slot, H115i if you have two. The beefier the cooler the less noise it will generate as it doesn't have to spin its fans as fast, and is less susceptible to ambient temperatures.
Thermaltake M360 Water Cooling Kit would be a good place to start.
If you not confident bending hard tubes then go for a soft tube kit.
The closed loop All In One coolers are mostly rubbish, the NZXT Kraken X72 holds about 60ml (2 ounces) liquid in the radiator for example.
Here is AMD Recommended CPU Cooler for ThreadRipper which have a TDP of 180 watts and 250 Watts: Thermal Solutions for Ryzen™ Threadripper™ Processors | AMD
So these would be more than enough for your Ryzen which has a TDP of 65 Watts but with a Max Operating Temp of 95C.
AMD sells the Ryzen 5 2600 bundled with a Wraith Stealth CPU Cooler (Non-LED).
youtu.be/yUCkQUG9bvY shows Deepcool's 360, 240 Castle AiO with anti-leak technology and fans specifically made for a radiator. It's the best-made water cooler so far.