A few tips right from the start:
1) Set smoothing on your faces so you lose the harsh polygonal shading, or "faceting," that occurs at the top elements.
2) Watch your triangle count - depending on the scale and final complexity of this object, it can be between 1k to 5k triangles (and would include interior assets). 5k would be for a fully decorated and rather large architectural model with several props.
3) Make sure you're using reference photos or drawings for a realistic construction - those stairs for example have to be held up by something. Also make sure that they are fully touching the ground planes and no section is floating in the air.
4) PlaneShift doesn't get along too well with double-sided faces, so make sure that you don't have them in your model. If this is something that only has to be changed when you use the exporter, then don't worry about it - I'm not sure how it works in Blender. Otherwise, just make sure that if something is flat but needs both sides viewable, both sides have a separate set of polygons.