Ok you can either consider a double din headunit, get a single din headunit and leave the cd changer, get a single din headunit and replace the cd changer with a pocket/storage tray.
Either way pull out the standard cassette player and have a look at the connections - if they are ISO then you won't need a connector (unless you have steering wheel controls) most aftermarket headunit will fit straght in. Check the aerial connector in case you need an adaptor for that.