If you have the factory nav, you probably have the TV tuner. If not, there is a TV tuner retrofit cable for about $35. The tuner is about $150 used. Search for X5 backup camera retrofit guides on Google. You need to run a reverse wire (active high I think) - the reverse lights might be a good way to go. Again have a read around.

A BOA DVD player is a good quality brand (BMW OEM) that would be a choice against the more expensive Apline. BOA 85700 even supports USB. You have no sound because the IR headphones are missing. You may need to spring for some Alpine IR headphones from eBay. Do check first how the audio works. I believe there is simply a TME-750 brain somewhere that takes the audio and video and converts the audio to IR, might also be RF. Take pictures of the boxes you find around the TME brain. It may have been a dealer fit option and a fairly scruffy install.