This is just my opinion having owned both digital and analog scopes.
Analog scopes are much, much easier to use at audio frequencies.
10-20 MHz scopes are really cheap. Here in the US, I think I can pick one up that is working for maybe $20-$50.
This is my opinion only: I saw some Ausie ebay for that BWD scope and I think the prices they want are nuts.
Make sure you can get the correct probes usually the scope has the capacitance of the probe next to the BNC input. Match it closely with the probe.
Almost always you would use 10X probes to sample the signal. Don't direct connect anything until you are comfortable with your equipment, even though scopes like your BWD have direct connections.
I have spent hours fixing the inputs of scopes that have been blown. ( I would never admit to doing this myself, however, HA!)
Again probes are really cheap.for low freq scopes. Chinese probes are mostly just fine.
Try local hamfests or whatever Craigslist thing you have available.
Turn it on and make sure you can see a good trace. Brightness counts.
Most scopes have a little connector for a probe on the front panel so you can observe a square wave. If you can, try it on the scope you are buying. Note the Cal on the BWD
Get the one with the biggest screen you can.
The Tektronix 465...475...485 are really good scopes and are probably the benchmark of analog scopes. But getting one in good working order is pretty difficult.
These things are 100 plus MHz, so you do not need them, I have one. Complicated and hard to fix.
I used to have a Tektronix "School" scope that was 15 MHz, had a really nice big screen and was just great for audio. Also hard to hurt it since it was made for use in schools.
Kinda looked like your one channel BWD 506, nice big screen. It might actually be a close copy, lots of folks copies lots of Tek scopes.
Here is a nice quote about BWD : While the use of commonly available components to build equipment had the effect of limiting performance, it means that BWD equipment is still serviceable, making it popular with Australian electronics hobbyists.
I have been disappointed in the digital scopes analyzing audio. Maybe I just have not tried the right one. It takes too long for them to "acquire" the signal and most of these scopes are high freq and need a bandwidth filter to adequately show audio.