I'm a wimp. I clean it first, then make repairs that were obvious from cleaning, then try to sew with it followed by needed repairs that were discovered by trying to sew. Any electrical (rewiring) I send to my OSMG.
If a machine is too filthy I'll pass on it (unless its cheap and a model I've hunting).
If it's missing key parts that are hard to replace, I'll pass on it.
If it uses hard to find needles, I'll pass on it.
If it looks like its going to take a lot of work to get up and going and is a common machine, I'll pass.
Would I be willing to buy a machine someone has already worked on? If the price was reasonable you bet! I enjoy using the old machines more than rehabing them. In my area it's hard to find rehabbed machines at a decent price. Example: last summer when I was in my OSMGs business he had 6 featherweights. 2 white others black, ok to good condition, only one had the case, 2 had original manuals and some of the accessories. They were just common, ordinary featherweights, he was wanting $450 to $650 for them. (He still has 4 featherweigts- dont know if they're the same ones or different ones.) I have found a guy wih reasonable prices and good machines but he's about 4 1/2 hrs away.