As far as a rust repair, thats not the worst spot. The rain channels are kinda a pain to work with, even worse when its rust your trying to remove. I would try to remove all the rust and if needed, put in small patches with some polyester enhancement material. The only thing about body filler is that cheap filler doesn't hold up to moisture as well as some of the non-tack stuff like evercoat or even Bondo Gold.
I hear people say "Bondo is a cheap shortcut", "it falls off", "if the body is so straight, why use filler?". Its a few people who don't take their time and make ugly/poor renishing jobs that forces the rest of us to explain that body filler is used at the factory by all companys. It is used to perfect body lines and contours. Another thing people say is you can only go to 1/8 inch of filler. In fact you can go to 1/4 or more if you layer it many times. My boss uses HEAVY filler and the whole 3 years I've worked for him, has never had a customer come back due to it chipping off. You can't even tell its thick filler when layed and surfaced right.
