Once you upgrade you cannot return to Factory Settings. As you know there are no windows 10 drivers for your model so reinstalling windows 7 is a good idea but you will have to use Dell installation media.
 I have Dell Inspiron 1558 originally running 7. I installed Windows 10 some time ago, keeping Recovery partition untouched. Now I'm trying to restore to factory defaults. When I go through F8 - repair - there's no "Dell Factory Image Restore" choice?  
No FACTORY IMAGE RESTORE choice