Which property would you buy?

Phew, difficult... both plots only have rather narrow sides, so depending on setback areas and such, you probably have to plan the house and carport offset anyway. I just want to mention that you can row really wonderfully on the Mittelland Canal... well, I'm a bit jealous ;-)

And you can enjoy cycling and walking! :-)

Where can I find the building envelope? The development plan doesn't say anything about it at all...

The building envelope will probably be the entire property except for the normal setback distances at the front and back. Since the city villa with 9 x 9 seems fixed, I am almost leaning towards 3. I think narrow long plots also need narrow houses; otherwise, everything feels too cramped. But try drawing your house on both, then you'll get a gut feeling for how it could turn out.

Exactly. Draw both plots a bit larger for yourself, and your house as well, then cut out the house, so you can move it back and forth and see where it fits best. You can do the same directly with a garage/a carport.

I would like to approach the property from the east

I assume from the house numbering that the municipality expects the access to No. 6 from the west side (?) How is the terrain there?