Nuendo has a format that can be read by Pro Tools so you should be good to go. One thing about sending your material to be mixed AND mastered, that will get very expensive if you are also needing mixing done, and that can take an awful long time unless your tracks are near perfectly mixed already. If I were you I would take time to mix my own material and then take it to a respectful skilled mastering engineer in your local area and see about mastering the entire project. Mind you if your not sending this material out as a demo or trying to really push it I wouldn't bother mastering it at this time.