I assume that your MiniDisc machine must have a digital output of some kind or another? So you would be wise to use it so as to limit adding any additional noise to your final recordings. Try looking at the Roland ED UA-30. This has both types of digital inputs and outputs, plus various analogue in's and outs too. It connects to your Mac via USB. But your Mac will also need to be running the latest Mac OS 9.0.4 for the device to work.
It's not the cheapest solution, but it's not the most costly either. Yet it should enable you to get top notch results.