1. I would say pick higher frequency of the UWB range as a adaptive frequency. I would suggest pick 15 GHz.
2. You want to make sure that all the waves get absorbed at the radiation box . If you use regular radiation boundary, use freq at 17 GHz.. FYI, if you are using FEBI boundary you can shrink your box to lambda/10.
3. Discrete sweep will give you most accurate results because it will treat each frequency point as an adaptive frequency. It will take longer time to finish the sweep. If can use fast sweep or interpolating sweep, they are faster than discrete , they will still give you good answer. Read the help menu for exact theoretical explanation ..