Hi and thank you for your post.
You cannot just look at the analogue bandwidth on its own, you would need to look at the sample rate as well.
In terms of using it as a spectrum analyser you can measure frequencies of up to 25MHz because of Nyquist.
In terms of measuring a signal you need to sample at least 5 times as quick for a sine wave or 10 times as quick for a square wave. The more samples you take the better your waveform will look. So 20MHz would give you a basic idea of what they waveform may look like, it really does depend on your requirements.
Another factor to look at is the sample memory, please have a look at the following link for more information:
http://www.picotech.com/applications/os ... orial.html