Finding the input bit rate of an analogue input
Theoretically speaking you need 2 samples per second for 1 Hz of bandwidth (regardless whether the signal is centered around DC or not), so in this case a sample rate of 1 kHz would be enough.
Practically speaking, you always need more since the theoretical requirements is based on the assumption of an ideal low pass (or bandpass) filter.
The correct choice of sample rate depends on how the band limiting is achieved and what bandwidth you are really interested in and what noise and sampling accuracy you need. Most audio systems use 44.1 kHz or 48 kHz to achieve 20 kHz usable bandwidth.
Another consideration is that you already have some digital inputs. These are derived from some master clock anyway. If you can use the same clock to sample your input, you can make the subsequent processing a lot easier.