+1^ Yeah, defo get a decent 550-650w EVGA G2/G3 unit, Seasonic S12II, or Corsiar CXm. They are quality units with 5 years warranties.
On the 144hz 1080p thing. I think you should manage your expectations. The GTX1060 is a great 1080p GPU, but at 60hz/FPS. There are of course some games you will be able to hit 144hz in, but mostly non-demanding games, but things games like Fortnite should run like a dream. Games Like BF1, COD WW2 and the like, will not play at 144hz/FPS with that card.
With all of the said, the 8400 is a great midrange gaming chip, and offers almost equal performance as an 8600k at stock.
Ram wise, i'd select a mobo first, then get a set of DIMMS that are on the QVL for the mobo you choose. That way you have a compatible set of DIMMS that have been tested to work with your mobo.
If you go the ram first before the mobo, you then have to find a mobo to support it, which rules out some nice choices for your mobo. You're kinda in reverse in that sense
edit: depending on the mobo you will have different levels of native support for ram. Some will allow OC ram, like 3000/3200 mhz etc. Coffee Lake runs at a native 2400/2666mhz speed. You don't necessarily need to get faster ram. If you were buying a Ryzen system, then I'd say yes, get faster ram. Intel systems don't react in the same way to faster ram as AMD systems do. This is because Ryzen uses the Infinity fabric, which when used with faster ram, actually has a perceivable effect on performance. Intel have a more rounded approach, so faster ram (although it will affect the system in a positive way, with small increase in performance) is not that important. Where it most certainly can be for a Ryzen system.
So when you've chosen your mobo, look at the QVL and get a decent set of 2666mhz DIMMS, and you're good to go. That IMO at least.