Thanks for the feedback!
The chain was pretty clean, and so the cassette and the chainrings. The rims are in excellent condition, and so most of the components (all of them original). The bike was certainly not used a lot. The guy who sold it to me recently started riding it (it was his father's bike) but was looking to replace it for a road one since he does mostly that.
As it was in great condition I rode it already for 200-250 km, including a 60 km MTB ride in wet & snowy conditions. Then I checked and found that the chain (original one) was a bit stretched, but not too much (my 0.75 gauge does not go through).