If you take a pic we can tell you exactly what you need but if nothing's there except the loom you will need a mk 4 nav (it will be the only one with DVD written on the face of it ) a bm54 (this is the radio) and lastly your CD changer
Prices vary a lot but expect to pay around 250 for each module and anything less is a bonus , CD changer is around 40-50 quid tops
Ps if you want to keep costs reasonable you could go for a mk 3 nav
Here's a mk4 http://www.adverts.ie/4944008