If all goes well, you can expect to have your new website completed and online in around 4 to 5 weeks. Generally speaking, copywriting is often the biggest stumbling block to completion, so having at least a draft in place will keep the project moving along. Once the website takes shape, draft copy and placeholder images can be updated.
If you haven't already done so, then you will need to purchase a domain name (eg: www.mybusiness.com.au) and hosting. There are lots of options out there but a general "business" hosting plan will be sufficient for most businesses needs, at a reasonable cost per year. If you already have hosting, then it is a simple matter to replace the old site with the new.
You don't need to! My experience has shown that leaving you to manage a complex CMS (Content Management System) in which to do your own updates often ends in frustration and wasted time. By leaving updates to me you'll be getting the benefits of my design experience as well as saving your own sanity.