Something to add: when I applied in Berlin, the 'sponsor' was no issue, but they wanted a detailed itinerary or 'program'. I.E. on which precise date you will be where and doing what. I pulled together the best I could from the Sudanese embassy website, Wikipedia, and Google Maps with plenty of historical sights and monuments to visit, and made sure to avoid and places too close to South Sudan, and that did the trick nicely. Just make sure the itinerary is as long as you want the validity of the visa to be.
Enjoy Sudan, it's a great country with some of the friendliest people around, great desert camping, and a feeling of safety you're likely not encounter anywhere else soon ...