Pocket Fender Flares & Xterra Rust

I just picked up a 2007 Off Road last week; my first Xterra. It has the typical Xterra rust over one of the rear wheel arches and inside the rear door wells.

Wheel Arch Rust

Obviously, I'm going to have to deal with that. I've never done body work before, but I was going to try to clean it, coat it with a rust inhibitor, bondo where needed, prime and paint it.

But then I saw u/rOXterra's pocket-style fender flares (pic) and had the idea that this might save me from having to drive around with a crappy DIY paint job. I mean, I'd still need to do everything listed above, but if the paint doesn't quite match or something it wouldn't be noticeable if it's covered by the fender flares.

This seems like a reasonable idea to me, but I wanted to ask if anyone with this style fender flares has any advice. Do the flares help prevent future rust by keeping those edges dry? How do they interact with the door seals?

I've got some underbody rust to deal with as well. Looks like I need a new sway bar, but I'm not sure about the leaf springs. I'm looking at putting in a 2" lift kit and new shocks, probably next summer.

Underbody Rust

Was thinking that, for now, I'd clean everything up and then hit it with an underbody spray, but I'm not sure what product to use. I live near Pittsburgh, so salt and snow are inevitable.