By "gnarly," what do you mean? Is it black and bubbly with some white crystalline looking bits? If so, NEVER have that guy weld stainless for you again. I'm guessing that he didn't properly back gas the backs of the welds. Fortunately, keggles usually hold the hot liquids in brewing, so your risk of infection from gunk hiding in the cracks and bubbles is very, very low.
If there is crystalline structure to the interior portions of the welds, it will be very difficult to clean with just a wire brush. You'll get any loose slag off of them, and they'll be clean enough to brew, but they will never look nice. If, on the other hand, there's just some discoloring, the stainless brush and BKF treatment will work very well.