I was looking at buying one of those welders to repair a Rv tank.
I decided to get it fixed by a local guy instead, and it was the best decision I made as the repair work cost the same as the repair kit, and after seeing the heavy duty equipment he used I would never have been able to get close to the same result with the small welder like you got.
I would say that a used set of plastics would be a better deal than repairing what you have, but you have already bought the welder. Try practice on a spot under the seat that wont be visible first to see what it will look like on the fender.