First off, do NOT buy right now.
AMD is dropping new cpus and Intel is cutting prices. Pretty sure the new Ryzen fixes the issue TP is referring to, however new Ryzen boards are EXPENSIVE. Older boards are supposed to be forward compatible, but I would wait for some reviews and people to get some experience first.
Air cooling is quietest at idle, water is usually quieter under high load, usually. Noctua can often be quiet regardless, they are also cheaper and never leak, but water lets you dump heat outside the case so that can be a big help.
Anyhow
Core I5-9400F Note the F, it's a 6 core 65 watt cpu. Less heat means less to cool, less to cool means less air flow, less airflow means less noise.
ITX board, you're pick, get m.2 NVME and a compatible drive.
Noctua cooler, Noctua fans. Make sure heat sink is compatible with mobo. See notes on case as wel!!!
Low profile ram, this ensures compatibility.
Corsair SF 600 power supply or DC-DC power supply. The fan on the Corsair will never come on under less than 50% load, so it's dead silent.
Alternate cooling
NZXT Kraken 52 water cooler (if it fits). These seem to be the quietest at the moment. Forget custom water cooling loops in a teeny case, no one has really managed to make them work well.
Optional:
passive cooled GTX 1050 no fan, no noise.
Case - RESEARCH,RESEARCH,RESEARCH!!!
Here is the thing with the ultra tiny ITX systems, NOT EVERYTHING WORKS. It's not a simple matter of buying an sfx psu and a compatible heat sink. They all have GPU size limitations, heat sink limitations and most importantly, air flow considerations. You cannot just slap one together and expect good thermals and noise levels. Find a community around that case and see what others have found works. Even with tons of research you can dump a LOT of money into a small system, only to find you still need to replace something else. Oh and fans aimed at you will be louder than fans with a panel between you and them, ventilation is good, but not of the fan is aimed at you.
Lastly, be prepared if you try for dead silent.
Besides costs, there may not be drive noise or fan noise but you may be surprised at how much noise just moving air can make, or worse, how much coil whine comes off computers. My desktop makes a bit of a woosh sound from 1-2 feet away but my file server gives off a pulsing whine from the cpu fan if it spins too slow or a droning whine if it runs a little faster, too fast and it howls all of which can be heard from across the room. There is a narrow happy medium but it's getting a replacement soon so it's less fickle. Others have reported loud coil whine.
Don't be afraid to let the cpu run warm. Many of us are used to 50-60c temps and letting it come up to 70, 75 or even 80c may get your hairs up, but it's still well within acceptable range. If you want silent, you may have to allow for some higher temps, especially in a tiny case.