Every real estate book or expert will tell you the main rule when buying real estate: location, location, location. Personally, I would also recommend to put aside the passive house aspirations for at least a while and focus on what do you want / need in the future as this is way more important.
Try to answer the following questions for yourself:
- Where do I want to live? (Countryside or City)
- Is nature important to me, or do I prefer short commuting times?
- For how many people should the house serve?
- Is it a pure investment or an asset I am going to personally also use?
The criteria you should consider for a passive house land plot are following:
- Orientation: South or South West as you will want to harvest the solar energy as much as possible
- Shade: Try to see if there are shade criteria which you cannot control and will have an impact on your life (for ex neighbours trees, a mountain, etc). Generally try to avoid places where there is shade more than 20% of the time of a daylight
- Micro climate: some places have microclimates which are quite different than the rest of the location. There are places where within a range of 2km there is a 3-4 degrees temperature difference
- Potential development: try to see if there is a risk that your neighbour will build a tall building which would obstruct the sun to shine at your house and therefore minimize the solar gains.