I would never buy a "set" of pre-designed plans. They just would have no chance of fitting the particular conditions of your site and your particular aray of needed functions.
I WOULD have plans prepared by a knowledgeable designer or Architect who worked with me to satisfy my needs and functions, existing conditions and budget.
If I had the ability to generate plans acceptable to the jurisdiction having authority, sufficient for permits, I would do so. If I had that ability, but not the ability to design the project properly, I would still consult a designer, for that expertise.
Bill (Designer)