You have three options.
The Infiniti dealer installed theater system, which would be the closest thing to the factory package. Pro is it has likely passed DOT crash testing (ie, it doesn't negatively impact whiplash protection.) and it will be compatible with your QX sound system. Negative is this is your most expensive option.
Your other choice is aftermarket. You can get aftermarket headrest DVD players installed at Best Buy. Just do a google search for prices and consumer ratings. You can get these installed for less than $1000, and they will look similar to the factory system. Not sure how they would impact whiplash protection.
Your cheapest option is to buy portable DVD players that Hang on the seat back. Phillips has an OK system, but you will have to deal with visible wires. You can get these at Best Buy or Target for less than $200.