I know you are anxious, but I would wait.
Over time pc parts tend to get better and cheaper.
Be prepared for a wait because of availability and early adopter shortages.
OTOH, consider pre ordering a i9-9900K from a source that can deliver and the price is near the suggested retail price.
Many will be jacking up the processor price.
Cases are a personal thing so pick carefully and buy what you want early, particularly if it is on sale or is hard to find.
You will almost certainly want a Z390 based motherboard. They will be expensive keep up with the announcements from your favored motherboard maker. I would not try to buy a motherboard earlier than necessary. New gen motherboards will have inevitable bios fixes which get sorted out after a bit of time.
If, like me, you favor M-ATX, then your choices may be limited or delayed.
On ram, wait until you have selected a motherboard. Then use the QVL list to pick a known good ram kit. Or, at least ram that is listed as supported for the motherboard on the ram vendor web site.
I hear that the high ram prices may be ready to go lower. Wait if you can.
Warranty if you buy early is no issue, ram will have a lifetime warranty.
Possibly, though, you will have questions about appropriate speeds.
Once you have decided on a case, you can pick a cooler that will fit.
Early on this would be ok, once you have your case.
I would not go liquid cooling if I could avoid it.
If you have a graphics card in mind, you can pick a psu.
Here is a chart:
http://www.realhardtechx.com/index_archivos/Page362.htm
It will need to be updated for the RTX cards, but you can figure that out.
You could buy a psu early, they do not change much.
Seasonic seems to have good sales this time of year.
I like the Seasonic focus gold line.
Buy only a tier 1/2 unit:
https://linustechtips.com/main/topic/631048-psu-tier-list-updated/