Dell won't support an OS other than the one it shipped. BEFORE you start the install, be sure you prepare WIndows 8 recovery media.
Once that's done, you'll need to purchase a full copy of Windows 7 yourself (NOT an upgrade version) -- most computer shops still sell these.
You can find all the WIndows 7 drivers for your system here: (choose WIndows 7 from the drop-down)