If you fit an radio unit, you will need to modify one of the connectors. The Denso has three 16 pin ones and the radio has one 16 pin and one 14 pin connector. See if you can get a head unit with a bit of loom.
There are different head units, you need the one that works with the existing amp. And there's no connection for any CD changer in the boot. If you want lots of CDs, get the head unit with the 6CD changer.