If you're starting with a relatively small budget, Magento offers the huge advantage that you can launch quickly with a minimum feature set. You can always improve and change your website at a later time, because it is open source software. Launching quickly means getting indexed by search engines quicker, and being able to test how to get customers quicker. The work on your store is never done. Trying to emulate the giants, such as Amazon, when you launch is impossible, so try not doing everything under the sun before you have even had one order. This goes for all aspects of your shop: marketing, sales, purchasing, customer support, and so on. You will only find out what's really important after you go live.

That said, your store should at least look reliable and work properly. Do not give your customers the impression you threw it online in a weekend. There are a number of ways to achieve this: good content, a nice logo, a well-picked theme, and more. A useful rule of thumb to remember when you start with limited means is to start with the absolute bare minimum of features on your site, and to start getting some order flow going as soon as possible. Only when you have real paying customers can you judge what you really need to grow revenues and profits.

If you already have an existing store or if you are going online with an already popular offline brand, the game changes. In this situation you have a prior reputation to uphold. This means you should perform a lot more analysis up front, primarily to match your existing business processes. As a result, your project will probably be longer in this case. However, the same rule of thumb applies: try to launch with the bare minimum of features.