I'll be honest, while I think #3 + Smart might be an overpay, if through some back channels Ainge could find out trading for Butler would get us KD, I'd pull the trigger.
And truthfully, I like the quartet of Thomas - Bradley - Butler - Durant more than if Smart replaced Bradley in that trio. Same elite defense, better offense. Throw Crowder into that mix and you really have a top flight perimeter attack on both ends of the court.