Fair enough, maybe 500 is a bit much, but I just wanted to say that its a significant amount and those two are the 2 items I will normally not go cheap on.
Judging by the parts mentioned this is a fairly high end build, so for a motherboard you would definitely go for one with a Z390 chipset and those start at around 150 and go up to 250 for nicer ones.
For a power supply you want to go for one with plenty of headroom and good quality. That means in this case at least 600W and a Gold rating. Something like a Corsair RM650 costs around 100 Euros.
Alright, slightly exaggerated with the 500, but its a significant chunk non-the-less.
But as you say, don’t forget: a case (don’t go cheap on that either), a good cooler (ideally an all-in-one water cooler, that’s another 100 Euros) and bits and pieces.
Lastly, if you really want your stuff to last and have some insurance that it wont break during the next thunderstorm or electrical spike (one of the main reasons electronic devices break), you should get a UPS (uninterruptible power supply). APC BackUps Pro are my favourite and are a good investment. Your equipment that is behind it is actually insured against electrical failure then.
But what intruiges me most is that @CeliaGC says “I know nearly nothing about computer hardware”. then why are you contemplating building your own computer? Even if you don’t assemble it yourself that’s still a pretty high risk to have a DIY computer, yet know nothing about hardware.
I would advise you to get a ready-made PC. I have a CorsairOne as my work computer and have to say it runs flawlessly since day 1. I have built many PCs myself, but if it’s for work long term reliability and serviceability beats the relatively small savings you get when building yourself.