Should we buy a second house and let it to our son who is renting?

[Brendan Burgess]

you don't know if you or Mrs SlugBreath will die first

Completely irrelevant.

The son owes the estate €400k.
If he is left nothing, he still owes the €400k.
If he is left more than €400k, that is deducted from it.

Brendan