I did a small test with the retrobright solution using regular old peroxide that you can get in the pharmacy. There was definite lightening. I have stronger peroxide in the garage waiting for me to try again. I need sun. Warm sun for it. It won't be soon.
With your serger, I would strongly recommend taking off all the panels you can see and oiling again. Everywhere metal rubs on metal. This is what a serger looks like when I service one:
http://www.archaicarcane.com/a-littl...-machine-sick/
So, yes, save the $100, but go in as far as the shop would have. Make notes / take photos if you need to so you can get it back together right, but get all the spots that need to be oiled.