[Greg Reid]

Yeah, I'd use a variable speed grinder and use a combination of 40~80 grit discs and a wire cup when you get down to the metal. Get at least a 4" grinder. You've got some filler in there and the rust will be beyond what you think. You won't get an honest assessment until you get the paint and filler out of the way for a fairly large radius around the rust you can actually see.
Not saying it's in bad shape...actually looks pretty good but you have to bite the bullet and get the damage out in the open...as painful as it may be. lol
I'd take all the paint off both quarters myself...repair as needed and epoxy them....then you can drive it for a while until you get ready to dive into it fully.
That's just the way I would do it. There's plenty of ways to handle it.