The kids are not that old yet and my eldest recently told me that she doesn’t like it when I sleep on a different floor than she does ;)
At 14 (or 11?) that can turn around quite quickly. As five (half) siblings, the kids might find it coolest to have a monkey and horse shared apartment and to have the old ones on another floor (?)
I believe the house will be too big for the age, both in terms of maintenance and as an overall property. Therefore, I am not planning it to be barrier-free. Otherwise, a sleeping and showering option on the ground floor would also be desirable. So I am planning for 20 years (current age is 40 years).
I find that a commendable attitude. The property is atypical, so you will need a lot of patience for resale.
I think the carts must fit in the yard and be accessible without any maneuvering.
Two shared cars for the kids should be enough. You take whatever is cheap and keep it clean enough so that the siblings can agree on it. Shared car management apps also work without users outside the family.