With 1/4"-thick angle, I wouldn't even bother gusseting. The angle would be at least as rigid as the 1/8"-thick stamped bracket. There is very little leverage on the angle legs for it to deflect with this application. If still worried, use 3/8" angle.
As far as corrosion, I'd say that the frame would fall apart before a 1/4" bracket would rust through, painted or not. As long as all the slag was thoroughly removed from the welds, there would be no accelerated corrosion compared to the other frame parts in the vicinity.
Anyway, I hate to see people get robbed. If someone came to me with this job, I'd charge $100 and have them out in an hour (no coffee or beer breaks for me). But then I don't have to pay insurance, overhead, etc., like AimCOtaco mentioned.