Slack is crap and matter most has mimicked all its worst parts.

I need it for normal desktop use. Email, coding, browsing (no gaming)

I need it to be completely without fans due to the location where I need it.

Yes i would say 1000-1200euro tops for the budget with option to add more ram later. Would you think i3 is enough? I am not too familiar with the newer generations

+1 for Joplin. Works really great and has some good plugins if you need to extend it.

Nextcloud

Switched a long time ago - it’s just soooo much easier and I never have any problems