IMHO - I would start off with the largest softbox & heavy duty stand you could find to fit with your budget.
Many great portraits are done with the single softbox - position it on the side and in front of the subject - meter off both the subjects shoulders (either with a handheld or in-camera) and adjust the softbox's position so that both shoulders meter about the same. You can meter before hand to get the proper position of the softbox and when the model ready all you have to do it shoot.
The larger the softbox the easier ( softer light ) it will be to get the shoulder's meter reading the same.