Personally, I’d avoid older stainless knives as the technology wasn’t really up to snuff.
In general for a carbon steel knife, (has a patina and/or rust), these are the things I’d look for:
1) Is it attractive to me? If not, I probably won’t ever like it - I’m shallow that way.
2) Is it well designed for the job I want it to do? (Handle ergonomics/material, blade shape/thickness, balance point, finger clearance, etc.,etc.)
3) Is it perfectly straight? I’ll sight down the blade several times from several angles. This includes the edge, and any twisting or waving that occurs at the edge. If not straight, I’ll pass.
4) Is there significant rust/pitting, and does it extend under the handle? If it’s very rusty, especially under the handle scales, I’ll pass.
5) How much wear (sharpening)/damage has the edge experienced? I’ll try to think about how much work/time it’ll need when deciding if it’s worth the price.
I’d highly recommend a finish of pure Tung oil and orange oil solvent (~1:1 ratio) for old, dry wooden handled knives, as this will provide a deep, clear, waterproof, food safe surface.
I’ll include some pics of a knife I recently restored a bit for a friend. It’s a good example of a knife that I would consider “on the edge” of being too rusty to buy used, as there’s definitely some rust under the handle, and removing and replacing a handle is usually more work than I want to put into a used user…
This handle was very dry and took up lots of Tung oil - probably 15 thick coats applied 20 minutes apart before the oil finally started pooling on the surface, then I rubbed it off with a rag (let it dry or burn before disposal to avoid spontaneous combustion) and let the handle dry.
Before:


After:

