Often the first one is not perfect so just start with what is awailable and make changes as you learn or find what you need. Many designs let you change the wheels if you decide to. I would newer buy heavy steel wheels, you can get second hand childrens bikes with alu rims for free or cheap. Many use them for folders, they are not all bad quality.
Maybe make a homebuilt "longtail" instead? Easy and really good solution.