Hi IngSTAR. Welcome to Geekhack.
There seem to be a lot of people who speak German, or a closely-related Germanic language, on here, so I imagine you'll be in good company regardless of your grasp of the English language.
I'm no fan of plastic either, besides polycarbonate, since it is pretty durable.
I don't normally do the kit/LEGO keyboard thing myself. You may want to post to ask about this in the
Keyboards section of this forum. It gets more traffic, and is probably a more appropriate place for the question.
Personally, I just buy whatever's cheap but still has good reviews without any immediately glaring design flaws and has the features that I want.
Have you tried to use a 60% keyboard before? I know I couldn't get by without at least dedicated arrow keys myself.
What switches have you tried? You may want to purchase a
switch tester before you make a choice of switches. Gateron browns and blacks are some of the better commercial linears and tactiles you could go with. I prefer Kailh's box linears and tactiles myself, ever so slightly, and Everglide switches are even smoother than those. I haven't tried any of the fancy/expensive boutique switches people seem to swoon over though either like NK creams, Alpacas, Holy Pandas, Halo switches, Kailh ink switches, Topre, etc.
I'm a clicky guy myself, so I don't bother to lube anything, not even stabilizers. That's another good question for your post in the Keyboards section.
For caps, you may want to post and ask in the
Keyboard Keycaps section.