I wouldn't focus on the very-well known function gens on the used market, since they are popular they are more expensive.
Look for not so well known models. For example, HP 3312a, HP 3324A, Philips/Fluke generators (e.g. pm5138, pm5139, pm5136), there are also always some Wavetek generators around, but I found it hard to get solid specs on them and skipped them. That's a very long list.
Patience pays, I bought my fg (50 MHz Philips) for c. 150 $ (excluding shipping etc.), but I had to wait a few months for a good offer... OTOH it might be hard to get spare parts for "unpopular" generators. But then again, spare parts are usually rather expensive... especially for HP, Tek, ... devices.
When doing audio stuff it might be worth to look for an additional sine oscillator or some fg with lower distortion numbers than general-purpose fun-gens, since those usually have rather high distortion (most synthesized fgs from the 80s-90s are in the 0.1-0.5 % ballpark). Building a good, solid, low distortion sine oscillator is btw. a nice project to do. I recommend it.