Crutchfield gets pricey, their website however is great for researching equipment and compatability with vehicles and what modules or interfaces might be needed to install the equipment. They usually include some free install pieces, but again still on the pricey end of things.
Sonic is also a great place to search and generally the best authorized pricing you will find.
When shopping you are going to find the same thing for drastically cheaper somewhere, this difference comes from a retailer being an authorized dealer vs a reatailer who is not authorized. What this difference means is that the factory warranty will not be honored on the equipment purchased through a non authorized dealer. Generally these non authorized dealers will plug their own warranty, or just not mention it.
as for playing movies while driving, that will all vary on which unit you ultimately pick, each has an individual method. As for brand selection they are all pretty evenly matched, basically look at the features you get for the dollars you spend, Kenwood right now is a really good bang for the buck brand.