It is a gem of a machine, I am a bit envious of your bargain find ;- )
The so called nicotine staining is usually traces of sewing machine oil darkened with age, and it usually comes of with gentle cleaning, maybe a cloth wringed up is soapy water. My Elna Supermatic had a lot of this staining, I polished it with a car polish (liquid type), and all traces of grime and staining went completely.
My green Phoenix is a bit worn, but wasn't as grimy on the outside, just very sticky internally and it took a bit more time to make it shift smoothly again. For that price this a 388F is well worth the time and effort. The tricky bit is replacement of gears and parts if needed, everything is possible, but it can take time to find or have them made. If we want (stretchy stitches / extra fancy stitch patterns) we usually have to accept a few plastic gears; my favorite zigzagger has a couple of plastic gears (an old Bernina 730).