Without knowing the brand of what you are looking at, I'm going to take a stab at answering you. Rust Converter is usually more of a paint type product that you use to coat the rust and stop it from coming back through. This does not remove the need for 'bondo' to make it smooth.

As for body filler, I would strongly advise against using 'bondo' brand filler. It is a cheap product, tends to shrink, does not sand well, etc. Spend the few extra bucks and buy 'real' body shop filler such as Evercoat Rage. I've been using it on my project and it is so much easier to use than the generic junk sold at most K-Marts and parts stores.