Purchase of an old apartment, renovation costs

For an apartment that is about 20 years old and is going to be lived in for about 10 years, I would definitely redo the electrical system

20 years old means built in 2001(!). The normative standard of the electrical installation back then largely corresponds to that of today. As long as the number of sockets/outlets is sufficient for the needs, I would leave it alone. If you want to be sure, you can have an electrician take a look at the distribution board, or replace individual sockets, for example in damp rooms (the mentioned terminal connections might be more oxidized there), but completely redoing the electrical system with breaking open walls and laying new cables seems to me, for this build year, like "using a sledgehammer to crack a nut."

Well, the recommendation is 30 years

Well, the recommendation is more like 30-50 years. In practice, often even longer, if you look at old buildings. I would only replace switches and sockets for aesthetic reasons. That is very cost-effective.

Well, the recommendation is more like 30-50 years.

Basically, power cables have a lifespan of 40 years. A DIN Schlagmichtot recommends replacement much earlier. Cable manufacturers sometimes only provide a 20-year warranty (at a maximum temperature of 70 degrees that the cables are allowed to have). The reason for the "short" lifespan is that the sheath around the cable can become porous.

BUT: I personally see no necessity to replace cables at all costs. In my 1960s house, modern 3-core cables were already installed, which (and we saw this during the renovation) were still in good condition. Meter cabinet, fuses, etc. are then another matter.

One could also have everything measured first, VDE inspection or that's what it's called. Large housing cooperatives and landlord-owned apartments in Berlin also have it done regularly (I think every 15 years, or when tenants change).