I am someone who has not used mechanical keyboards before, but I am curious about them since I read everywhere that they are great. The issue I have is that I obviously don't know which things I will like or not. For example the switches, there are a lot of brands, and they produce different switches. When I read about them, I see the familiar terms: "clicky", "tactile", but I have no idea what that actually means.
However, just starting to buy stuff to test it out will become very expensive, very quickly.
How do you start with this without spending hundreds and hundreds of Euro's at the start (and without knowing if you like it or not)
Since you're looking at non-ergo mechanical keyboards, you have way more options and flexibility.
I would get a quality hotswap board (Keychron boards are the usual recommendation, if you go Aliexpress you can find cheaper options too) and a set of any switches you like. Switches maintain resale value well (some even raise, because broken in switches tend to work better than non broken-in) so get a set you're interested in, daily drive it for a bit, and then see what you'd want changed.
You can also think about your current keyboard setup, what you like about it and what isn't working for you.
There exist external mechanical numpad/macro pads that you cna use if you wanna downsize the board. Then you can hook up the numpad only when you need it
Pro-tip: getting rid of the numpad is good for you long term. Num pad existence is associated with RSI pain. I would consider a 60-80% keyboard and a separate numpad that you can pull out when you need it.