On a scale of 1 to 10, how hard will be this to fix?

 

bondo and spray foam ? lol
1 bein easy, id say 8-8.5

 

By the looks of it that looks like your fender and the rocker under your fender right? If it is then get a new or a "new" fender and either paint it or have it painted to match if you want. The rocker you need to either POR15 it and paint it or grind off as much as the rust you can, prime and not rattle can, and then paint. You could also try and patch panel the fender it you cut all the rust off and match the body lines but it's easier to just replace it.

 

Yeah, that is the area. BTW, what exactly is the "rocker"? This happened (best guess) because the sunroof drain tubes drained down to this area. It's like this on both sides.
I plan to do some medium frnder trimming, so I will probably work with these original fenders.
Kinda sucks, this is the only rust on the Jeep.

 

The rocker panel (rocker) is a structural body part that connects from the quarter panel and runs under your doors to the the fender it is where the fender bolts on at the bottom. It connect the floor and all your pillars and the quarter panel together. It has an inner high strength steel part that runs in it for strength against crashes and rocks the outer shell is kinda weak tho. That is why rock sliders have bars that run the length of the rocker to protect it from abuse.

Google or Wikipedia can explain in more detail if you need it. I just gave you what I know from working in a body shop and going to school for auto body.