From personal experience (albeit ten years ago), you can never be too formal when applying for a job in Germany. The standard business 'if you have further questions' phrase is "Falls Sie weitere Fragen haben, stehe ich Ihnen gerne [per Skype] zur Verfuegung"

I would say 'Ist ein Arbeitserlaubnis noetig", I think. Though it would probably be better to look this up online (are you in the US? in the EU, you just apply for it when you get to Germany)

Hope that's some help.