I'm not really a facade expert either, but it sounds good (thick). Even Styrofoam already in there. I would do the windows and underfloor heating, add a little extra insulation to the floor ceiling as own work, and then go ahead and calculate the heating load. A heat pump should work without many additional extra things. Heat pumps are well subsidized, for window replacement you have to check yourself if there is anything available. In advance: Don't let anyone sell you a heat pump that's too big. And it must be modulating (can deliver less than its maximum capacity).