The major rust areas are:
Hell hole (under the battery tray)
Longitudinal (longs) frame members (especially pass rear near jack plate)
Rear trunk floor (especially where it meets the back panel)
Floor pans
Around the A pillar
But the cancer can and does crop up everywhere. Do a google search with 914world + "rust" + any of the above and you'll learn pretty quickly what to look for.
If you can't find a 914 person to go with you and the car looks good, take pictures of the above areas and post them here. Best of luck!