Cohabiting couples- contrary to married ones- do not, to a certain extent, have the same rights concerning inheritance: one could say no rights at all. However, there are several steps you can take so that the survivor, in case of the death of their partner, can keep a house they have bought together.
In the capacity of landlord, you can choose from among the different prospective tenants who put themselves forward. But this must not be discriminatory. If you do discriminate, you risk criminal sanctions as well as other consequences.