Well, there's no rack-mount arrangers but plenty of table-top models like the Yamaha QY-series. Honestly I would go with a sound module/SMF player like an old Yamaha QY-70 or a Roland SD-30 (I've started many OMB's with these). Or a laptop with a GM softsynth for SMF playback. That would allow you to use any accordian or whatever for the lead instrument.