Berlin Mitte and especially the area arround Hackescher Markt (the locals call it “Scheunenviertel” or just “Mitte”) is a hot spot at the end of his lifecycle of the gentrification process, very international, high rental as well as buying prices. Some good deals available but very rare. Demand is by far outreaching supply. The area has no real ventil but all alternatives as there is (only) Prenzlauer Berg for e.g. don’t have the same access to the city center of West Berlin. The twao main S-Bahn routs are crossing here (S1 = Mitte to South-West and Potsdam) and the “center line” driving trough Mitte into Friedrichshain as well as to the Center West (Zoo, Savigny Platz). The desity of clubs culture fashion shops is very high. Which is problematic because there are lots of hostels and tourist destinations. The micro location of your investment is very important in Mitte. Make sure there is no Hostel or Club next door (use Google Maps).
Prices for prewar flats (rehab) starting at EUR 2000/sqm and go up to 6500 Eur/sqm for state-of-the-art rehab roof tops.