You'll be able to sort a ticket out in Hannover no problem, you need to take a train from the airport which is E2.80 and takes about 15mins (think they run at 06 and 36 past the hour) train to Wolfsburg takes about 45mins and costs E23.50 but I'd say you're better staying in Hannover for beers for as long as possible, trains are predictably reliable so wouldn't worry too much just don't forget to time stamp your ticket or you'll get a fine ticket or not.
Personally I'd stay in Hannover as long as possible for beers pre/post match, there's a decent sports bar at the back of the train station and the infamous Jack The Rippers out front (English bar) or go to the Paulanner Bar (main train station exit and then left) to get the Bavarian experience or go opposite the Opera House to Oscar's for a more Hannoverian experience.
Planty of decent cheap hotels around and about and I would expect to pay more than about 40 for a night for a reasonable 3 star.
Spend a lot of time in Hannover and it's no Hamburg/Cologne/Berlin/Munich (the locals will blame 'you' for the lack of old architecture with their tongue wedged firmly in their cheek) but has it's own charm and is pretty friendly, remember being there when Portsmouth played Wolfsburg and being pretty embarrassed by the constant Dambusters/God Save The Queen chants but to their eternal credit not one local took the bait, it did kick off in Jack's that night and while I know we are better than that make no mistake, the local police take no prisoners, if you fight, cause damage or are disordly, they will kick the shit out of you.
If you have the time and the inclination take a walk to the Machsee and get a drink and a currywurst on the lake or if you're feeling hungry go to Paulanners and get a bayernteller platter (though not if you're vegetarian or Muslim....).
