To follow up what NItroWill said. I thought you were referring to just a screen. A dedicated multimedia and nav system will run you about 1000.00. Any of the Pioneer Avic systems will do what you need but you aren't gonna find a system for less than $800.00. You need to find out what you need in features and I'd be glad to point you in the right direction.
Yes you need an account to buy on eBay. You are going to find your best deals there most of the time. There are online car audio dealers that have soem good pricing but you have to look around.
-Lil'