Here's the problem ...

Mold and mildew form wherever there is moisture. So it is on the back of every panel, behind the dash, on the wiring, under the headliner, under the carpet, inside the seats, etc, etc.. So when you tackle it, take as much apart as you can. Any mold you miss will spread if conditions are right. If the car smells of mold and mildew, its there!
Once you've cleaned it to your satisfaction, you'll need to keep the car moisture-free while in storage so you should invest in a de-humidifier for your shop. The worst thing is humid summer weather.

Also, if its been there a while the mold and mildew is going to leave a very distinct stain that is impossible to wash away. Armorall or a similar product will mask it.

Good luck !!