I'm thinking of building a django application which is like an online shopping mall similar to eBay and Amazon. My business model is somewhat like that of Shopify or Volusion, which means I will build the app as a hosted ecommerce solution/SaaS that offers different plan packages for the users to sign up to and have a store.
I have never done this kind of things, so I am quite frustrated and clueless at the moment.
Please, if anyone knows how to do this kind of stuff, give me a hand or at least point me a direction where I should go to find information on this.
Can I give you a word of advice?
Pick a project that you know much more about and do that successfully first. Being a successful entrepreneur is a huge learning curve. It is not about implementing the first big idea you have and striking it rich first time. Even Mark Zuckerberg (young as he is) worked on a 3 or 4 smaller projects first - before he launched Facebook. This enabled him to learn all he needed to know to start and grow FB successfully.
Trying to pull this off when you know so little about it is unlikely to succeed. Pick something you can do now and do that - and learn.
Volusion and Shopify are complex products that require a team of developers to build and maintain. Not only are there many aspects of their products that you most likely have no experience with or knowledge about, but also how will you market your service, and will you have enough credibility for people to use it? And why do you want to re-invent this wheel anyway? All important questions.
Regarding your first question, I would prefer a shared hosting solution first and then to scale up to dedicated servers. Since you are into SAAS, try to use the EC2.