What exactly is the Sixers plan?
Draft Simmons who can't shoot, trade Noel or Okafor, then play whichever one they keep next to Embiid at the 5 with Simmons at the 3?
Then have Dunn at the 1 who is an average shooter?
That team would have the worst spacing of all time.
You have to remember this roster is going to be incomplete this year no matter what. Next year they'll have their own lottery pick, the ability to swap with Sacramento, and the Lakers top 3 protected. So they'll be adding probably two more lottery picks next year to complement what they are putting together now.
As for Dunn, they need a point guard, and independent of everything else they really like him. They need shooters also, but that's a separate point.
Can Dunn play with Simmons? They likely see Dunn as a very good perimeter defender and very good at running the break and transition offense. That actually pairs well with Simmons.
You could argue Ingram and Dunn fit better, but Simmons is the best player in the draft so you take him. And I don't buy that he can't fit with Dunn.
They probably play Simmons at the 4. Saric probably is also a 4. Embiid, Noel and Okafor are basically 5s. So you trade 1 or 2.
Simmons can probably guard at the 3, Noel can guard the 4, that's the important thing.
After that, work in progress. They probably will sign or trade for a veteran backcourt player.