I do a lot of floorplans for people, although not nearly as many in the most recent years. Most of the time, it will start with a plan from online or out of a book. When I make my first drawing based on the dimensions shown on the plans, many of the rooms are not the correct size and the dimensions don't add up, and I assume this is what the plan makers do to make it more difficult to "steal" their plans (and I don't blame them).
I've always thought that the most difficult portion of doing a set of plans is balancing what the outside of the house "needs" to look like, either for curb appeal or to blend in with it's natural and man-made surroundgins, vs. what the owner "wants" the inside of the house to be like. A lot of times, I'll try to get the owner to start with a flow diagram rather than a floor plan, so we can focus on what is important in terms of day-to-day activity. I've noticed too many times an owner will want to focus on details right away and forget about the big picture.
I always give my recommendations about making a house more universal, for resale purposes, and I think in almost every case, the owner has assured me that this is their "forever home, we'll never sell or move". I would guess over half of the homes that I've designed have been sold. My recommendations are very much inline with other's: entry closet, and plenty of closet space in bedrooms, trying to accomodate 3 beds/2 baths if possible, nothing too "unique" interior/exterior finishes, etc. Having said all that, I will ultimately do what the owner wants: one house was a basement house, and none of the bedrooms have any closets: they wanted to use wardrobe cabinets. I will say that in this case, the original owner remains in the house, so at that point, this is not an issue.
I would go with a local designer rather than a set of online plans, just for the variability involved in house design. Whatever you choose, good luck and take care.
