Since you have the Essentials Edition already, it would probably be cheaper to just buy the DLC that comes with the second disk that comes with the disk that you already have (the second disk is included in the regular edition of the game and it contains 3-4 DLC packs, but are available on Xbox Live). I would also recommend that you buy the Season Pass after that, which includes a lot more awesome cars, but that is also another $30. It just really depends on what cars you want and how much you are willing to pay. But, I would highly recommend that you buy the DLCs that come on the second disk.
Thank you Mike! As a new Xbox360 user, I have to ask the dumb question of how do I download the DLC packs and what is the Season Pass? Just so you know, I don't have the paid subscription version of Xbox live but do have the online connection to it so that I can still get demos, etc. Thanks for your help!