Where should I place stairs and is it possible to get the stairs to spiral?

Spiral stairs (rounded shape) isn't possible in sims4 unless someone has made a CC with animation (i.e. rendered from sims3). With cheats you can somewhat do square version of spiral stairs, however, I don't know can you get them "pretty" inside a house. Another option is floor by floor but it doesn't look all that great, if you are after spiral stairs.

Since you build with money your Sim earns, I'd recommend using the plain straight stairs without trickery... the placement... depends how many staircases you plan on adding. If only 1, then I'd put it somewhat to the middle. With a large house/lot I personally prefer (if possible) at least 3 staircases: middle and 1 on both sides. For a grand manor you could just use 1 broad staircase somewhat in the middle. Think of it this way - how are you planning on building the rooms and how easily would you like to go to them with your Sim and that will give you the answer as to where to put them logically. Style, creativity and artistic looks are another matter entirely where comfort and logic doesn't always apply.