Posted by: @8042franz

But the frame only has surface rust the leaf springs are barely holding on it

Are you describing the leaf springs themselves, or the mounting points where they attach to the frame? The mounts are replaceable, they're called leaf spring shackles and mounts. Providing you have some hardware around the house, and the frame itself is actually intact, these are replaceable.

The Explorer is basically a Ranger with an SUV cabin on top. You don't want one of those leaf spring shackles to break, especially while driving. You can lose control of the truck. It will also potentially bust a hole into the cabin. Chrisfix has a good video on how to replace them. Both rear wheels need to be raised off the ground.  

(19) How to Replace Leaf Springs and Lift your Truck - YouTube

Body panels you can find in junkyards. Personally, I wouldn't go too far with it. It's never going to be worth anything. If you want a beater, it may have some life left, but that's it. 

Posted by: @8042franz

Would it cost more to fix it all then the vehicle's worth?

by the sounds of it, yes

Spark plugs and a muffler are not a big deal. Just how bad are the leaf springs?

Sounds pretty far gone. Unless you have the time and welding skills to make it your project truck, I’d dispose of it.  Any running and driving 4x4 will have some value, but it is hard to believe it’s worth the cost of rust mitigation, by your description.