I used the 4 inch wire wheel (braided) on my truck with a 4.5 inch grinder. It does the job well. The deciding factor is how you will be painting the frame. The wire wheel will get most but not all of the rust so I used Zerorust as a base coat on my frame then covered that after a full cure with high temp engine paint. It has provided a good hard finish. If you plan to expoxy prime your frame you might be better off getting it sand blasted to remove all rust. It sounds like from where you will be it will not be that much further to get a bare frame.

The 4.5 inch grinder is very useful. I have one with a wire wheel on it, another for grinding and another with a thin metal cutoff wheel. Saves frustration of having to keep changing back and forth. Good luck.