Nowi: +Spd -Res or Neutral

This really will come down to what you want your finished project to look like. If arena is important to you bear in mind that Res is a super bane and will affect you arena score slightly. With that said a +Spd Nowi has massive defensive and offensive capability. She's hard to consistently double and if not one-shotted hits back extremely hard due to her adaptive damage. If you aren't overly worried about arena than stick with the +Spd one.

This is my +SPD -res nowi. As you can see I really focused on getting her speed as high as possible without mentioning buffs. Given the build you are looking at though I would say go neutral. Personally I would have actually preferred +SPD -hp but luck was not on my side.