Sorry, I don't understand your first question, if you are flying 135 cargo you are always going to be on a IFR flight plan even if it clear and a million. Yes, they do have flights into Sioux Falls from out stations.

“Not all 135 is IFR”, is the short version. Some operators, in an attempt to find someone willing to fly a piston powered cargo bird, will hire a VFR 135 captain at 500hrs. Ameriflight had PA-31’s doing this in the Southwest. Sounds like you have 1200hrs and meet the IFR captain mins? If so, I’m saying that you could go right into a turboprop. However, if the twin piston fits other needs in your life, that makes sense too.