A tracker is a music app that works a little different compared to the usual midi-sequencer. You (usually) work with samples, but softsynths are common too these days.

Here's a nice one, and completely free: http://www.buzzmachines.com/
Another, also a famous one: http://v75.vweb.citenet.net/it/features.html

If you're buying really old keyboards, always try out all the knobs and sliders etc. Most of my synths were bought s/h.

-tek