You need to assess the risks practically. Cats do roam, but some have a relatively small patch, as was proved in the TV programme where they tagged the cats. You CANNOT catproof a garden 100%! We have 3 cats and they will and do roam. However - if you are willing for Missy to wear a collar (I wouldn't be, if she's going out, they are more risk that they are worth, IMO) then you can buy a GPS device so you can always track where she is. They are not cheap but those who have them say they are worth it.
I had house cats in the past when I lived in a flat on a busy main road, but tbh it's not natural for a cat. They love the open air and chasing leaves and spiders and just looning around in the sunshine. If you keep her indoors, you'll be constantly door-dodging - she'll always want to get out. Go out with her for the first few times, then bring her in until she gains confidence. Take her out in the garden on a harness at first if you like....
If you are going to let her out - get a locking cat flap. She'll soon learn to use it. I hope that you have had her microchipped already (essential for a cat that goes out), and you can get a catflap which programmes to the chip, so only she can get in.
I hope this helps!