Conversion of concrete basement into office and guest room

We have bought a semi-detached house and are renovating diligently. Now we are considering using part of the basement as an office and guest room. The floor has insulation and on top of that is screed. The walls are concrete shell elements that have been filled with concrete in the middle. Two existing basement windows are to be replaced with regular windows. Since the walls are very cold, we are wondering what we can do about it to avoid enormous heating costs. A radiator connection is available and a small wood stove is to follow. Is there a recommendation on how to insulate the basement cost-effectively?

Our concrete basement is insulated from the inside with aluminum-coated styrofoam, I don't remember the exact designation. The cables then run concealed in the walls.