I'm not German (Dutch actually) but I live near the German border. When in Germany, I usually speak German, but sometimes in English. Most of the time when they think my German isn't good enough. I'm terrible at the German grammatical case, what makes some conversations rather amusing.
The V W mix-up is more of a sound somewhere bitween the two. It's hard to explain via text.
The English "TH" sound is hard to pronounce for Germans (and also for Dutch), usually pronounced as "Z".
The English "R" is also hard to pronounce for Germans. Their "R" is more of a growl.
For translations, I would use Google Translate. As long as the vocabulary stays simple without typical English sayings. Cross check (cut German and have it translated back to English) if you want to be certain.
You can listen to interviews with Michael Schumacher or Arnold Schwartzenegger. The later is actually from Austria, but the language is about the same as Dutch.
Beer Garden or Biergarten are common in Germany. You might want to look into a specific brand of beer for your character. Germany has many beers.
Speaking in a forreign dialect is a bit of a bluff. But what if someone calls your bluff and start to speak German?
I speak Dutch, English, German and a little bit of French. The French (in general) are fond of their language and usually don't speak another language. Some of them make fun of my French, pretending they don't understand what I'm saying. When encountering such a person, I usually give him this treatment:"Spreekt u Nederlands?" "non", "Sprechen sie Deutch?" "non", "Do you speak English?" "non", "hable Espaniol?" (I don't speak Spanish, but that's the only sentence I can pronounce fluidly.) "non" After this, I switch back to my terrible French and usually they switch to English.
Only one French speaking person called my bluff and started to speak Spanish.