Sorry - free site on a free domain = unprofessional
Whats happens when it takes off ? you dont have your own domain so your marketing is all going to weebly, hat happens if they go bust you will lose everything.
Its not all doom and gloom regarding starting with a site such as this. All you have to remember is when you move to your new platform, 301 re direct all your previous links and content. You wont loose all the hard work regarding SEO then.
Nick