sakurapeach,
As for this particular concert, you never know if tickets will be available at the door or not. That's what it says on the O-East website.
http://shibuya-o.com/east/2012/09
At this moment they are available, but the gigs in Iwaki and Sendai are already sold out. You can book tickets now, but I'm not sure if you can do it from overseas. Here is the booking site, anyway.
http://eplus.jp/sys/main.jsp?uji.verbs=GGWP03_search&uji.id=search&uji...Or you can phone the promoter direct.
DISK GARAGE 050-5533-0888
Sometimes one drink is included but not for this one. Drink prices may vary, but they're usually a few hundred yen a cup.
A lot of venues, especially upon popular concerts like these, make people queue at the door depending on their ticket number. The staff usually takes good care of you, so if you notice someone shouting or if you notice people getting in line, show your ticket to a staff or a fan and that person would hopefully guide your way. In other words, try to show up ahead of time.
Also note that for this concert, they say that they might limit the number of people getting in if it gets too crowded. This usually means that you may be allowed to go in only for a certain period of time instead of being there for the whole festival.
By the way, with uploads and SNSs being so popular and effective nowadays, artists and promoters often let fans take photos of their performances. For this particular concert, I see no notes telling you not to bring in recording devices. So you might want to ask a staff about it before you lock that camera in.
But here is a floor map of the venue, just for reference.
http://shibuya-o.com/floor-guide#east
Hope you have fun.