I've just tested the CPU Temperature. It was between 19 and 21°C. Then I've started a Game which caused it to crash at 40°C
Where are you getting these temperature readings?
19C isn't physically possible for a core reading, even at idle, unless ambients are sub-15C. Still, even with a significant offset, it's doubtful that CPU temperatures are responsible for that crash.
This Power Supply Unit seems to be very good
https://www.amazon.de/dp/B07VHFDVK2/ref=cm_sw_r_cp_apa_fabc_GYwYFbBSW2P5W?psc=1
But how do I find out if I should choose one with 750W or lower?
Barely a review, but it's the most I could find on that unit:
Page 3 - Physical Look - Inside
aphnetworks.com
That's a Gospower-built budget unit (this is a good example of why you can't always go with branding...that's the first Gospower unit I've seen and makes at least five OEMs that Coolermaster has used in the last five years). Without someone attaching a real load tester and a scope to it will be impossible to know how it performs. That said, the components seem a bit on the cheap side.
It's probably serviceable, but I suspect there are better PSUs for the money. Also, quality is more important than the listed power output, within reason, but 650w should be plenty.
I skimmed the German Amazon site and the only PSU I recognized that I would recommend without reservation in a vaguely similar price range is the
Seasonic FOCUS GX-550. It's a bit pricy for it's output, but I'd rather have a 550w Seasonic FOCUS GX than even the 750w version of that Coolermaster/Gospower series.