Adopting a stray cat as a yard cat?

This little bugger has been hanging around my SO's workplace. She's absolutely attached to us and would show her belly every time we try to pet her. And it's not a trap! She loves belly rubs.

Now we wanted to adopt her for a while and have been taking care of her, vet visits, vaccines and all. We really want to have her in the house but we already have a cat (who'd love some company) but house rules allow for just one.

We also have a yard which is cat-friendly with shelter, places to sleep in, climb and scratch, and not directly on the street.

We know she's used to her environment and our worries are that she'll run away and try to go back to her old place, which is right on the street where her sister perished by a car at an intersection.

How can we go about this?