The problem with second hand Old Skool oscilloscopes is that they may fail and be uneconomical to repair. You might not be able to trust the calibration, and the CRTs in well-used units can be quite dim (usually not a problem in audio work, though).
I would suggest a PC scope, they are rather inexpensive nowadays and you usually get all the goodies (like memory) that will be missing from lower end real scopes. For some very rudimentary audio work, just might get by with a software scope that runs through the audio card, I believe there are a number of free applications for that (perhaps not for Mac users, as they all have a lot of money to spend ).
Having said that, I have a Tektronix 2235 myself that I bought used about 15 years ago, and it gives great service!
EDIT: Yes, those PicoScopes are quite nice. Also, sites that sell everything Chinese have standalone mini scopes with LCD displays for less than a hundred bucks (the size of a cell phone). I don't think I'd want to use them for any serious work but they might do the job, and won't hog your desk.