Real accordions are either wonderfull to record, or they wheeze and creak really badly. They also move quite a bit with bellows expansion, and the felt pads on the right keys often clack badly if the player is a hard hitter, and the ones on the left side make a sort of kerthump. stereo can work, but emphasises movement which may/may not be what you want. Brighter generally from the keyboard side and more bass from the left. Treat it a little like a vibraphone/marimba type instrument that physically moves. It has a defined left and right difference, that closes up as the bellows close. The idea of a 57 or 58 to start with isn't a bad start till you hear the instrument, when replacement with condensers could be good, if it's bright and in decent condition.