That brown is almost certainly nicotine from the machine being in a smoky office for decades..  Start with a strong detergent, then progress to a mildly abrasive car polish on a soft cotton polishing cloth.  Keep clear of the decals, as this may remove them.  It will take you a long time,  You will see the brown coming off on the cloth.  Do it properly and you will have a 'piano finish'.  Acetone would be fine to use on the metal parts.

Chris,

My go-to, all around cleaning solvent is denatured alcohol.  I do not worry about it around smooth paints and plastics.  But I will still do a test-patch under the machine or the inside surface of a removable cover.

For cleaning crinkle-paint on machines like the 1950's Smith Coronas, etc. I use a can of scrubbing bubbles and a soft white cotton rag or hand towel.

For stubborn work (like cleaning type-slugs, I will use lacquer thinner and acetone...but I have to be extremely careful to not get it near any painted surfaces and plastics and rubber.
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Good point, my mom always said, take good advices but don't forget to think - So I checked the ingredients or in this case use one where I know whats inside. Also a lot of detergents have high amounts of oxidizers so sometimes an old fashioned "full" (German term) detergent is better. That's what I used here. Warm water often helps a lot so yesterday just brushing parts with foam instantly turned the foam deep brown. Now all is shiny like a piano