My DDs were, DD2 more than DD1 (in fact, DD2 is in the anime/cosplay society at uni) and both have gone the the expo a couple of times. DD2 was also into drawing manga.
Definitely take water and snacks, the food there is expensive, especially when compared to a supermarket sandwich/drink/snack meal deal. From what DDs have told me, there's a lot to look at, a lot to buy (and some of it rather expensive), lots of people taking pictures and lots of people to take pictures of, that it gets rather warm and a tad crowded, plus the queues for tickets can be amazingly long, but it is worth it.
Not sure about car access, my DDs got the train (admittedly DD2 was 14 when she first went and got the train with DD1's 17yo friends, then next year DD1 was 18 and very used to the tube/DLR). I'd definitely check out the website and check what the venue says about getting there.
From what I can remember, they were both into Naruto and Fruits Basket, plus I think DD2 was into Kingdom Hearts, Vampire Knights, Bleach and Death Note, plus many more I can't remember. There was one that was something to do with a high school that had a pair of male, red-head twins in that acted a lot like the Weasley twins both DDs loved. Not sure what it was 