Once you start buzzing the rust down, it will be worse. Much, much worse. Been there, done that. However my suggestion to repair it will still work just fine. With some time and patience you can do a reasonable looking job of patching it yourself. I know not all of my frame repair jobs will win any awards for looks but they are strong. I did a similar type repair on my jeep to the rear frame rails and the subsequent heavy duty off-roading and ramping it over a small ditch has proven the strength of my repair!