i would advise to do the whole back part of the panel, the rust may not be showing but it will be by and even over the top of the filler hole on the inside.

up to you though but i would do it that way, dont wanna do all that work for rot to come back through in 6-12 months

this is what i put on my car

£5 from scrapy, and 100% rust free, well a little surface on a tiny dent on the edge (since be knocked out and ground up)