The D23 Expo is three days Fri-Sun. You can buy tickets to the individual days, but I definitely recommend going all three days.
Tickets can be purchased here http://registration.experientevent.com/ShowDIS151/
The event is very crowded, but daily schedules are released a couple of weeks in advance so you can try to plan your days. You don't have to pay money to go to the individual panels. You just have to know that anything you want to go to will have a line to get into so plan accordingly.
I have been to 2 of the 3 expos, and I had a blast.
Last time they had the event Disney Interactive had a big presence on the show floor, and they had a presentation on Sunday of the event. No known info has been released this year, but I expect the same.
The Anaheim Convention Center is directly across the street from Disneyland. So you can look up hotels in the Disneyland area online to try to find the best rates. Knowing this, you may also want to schedule 1-2 days at Disneyland if you want some Disneyland time. There are quite a few members on this forum attending the event, and we are planning a meetup so you definitely want to subscribe to that forum topic when we post more info.DIF D23 Expo Meetup 2015