When your business doesn't have in house knowledge on web development, you need an outside vender to help you build a website. If you found one that recommends Drupal, this is what you can expect from the project.
Analyze your needs
Websites are not made with a cookie cutter. Each one is carefully crafted to suit your needs. Depending on how much you're spending on design and development, you could be getting a bespoke site, or one put together with off the shelf parts and styles. Agencies usually charge a flat rate, so the price you pay depends on how much you ask for. A good agency will be clear about what you can do for your price, and advise you on making high impact choices.
Project plan
This is a projection of best guesses and not a precise delivery timeline. Making technical things happen on an exact timeline is not possible due to the unknown difficulty of each task. The best you can do is approximate how long things will take and determine how much time and budget will be available for each piece. The more you add to a project scope, the harder it will be to accomplish it. If you want a marketing site like apple.com, it will take top level designers and developers to put together, with experts in marketing and testing to perfect the message and display. If you have a modest budget, focus on features that will maximize value, and eliminate ones that distract from putting polish on what users will see most.
Sprint cycle
Most developers work in 2 week cycles, called sprints. At the start of the sprint, the team works with you to determine top priorities and next steps. They commit to what they believe can be achieved and schedule a demo at the end. This measured pace allows us to estimate how much we achieve in each sprint and maintain relative accuracy throughout the project. We estimate features in points, which represent relative difficulty. It's easier to compare features to each other and determine if one is a little or a lot harder, than it is to come up with an hourly estimate for everything. We tally the points on completed features each sprint to determine the velocity and use this to set the target number of points to complete in the next sprint. You can use this calculation to determine the number of sprints needed to complete the features in the project. If you prioritize the most important features early in development, by the time you exhaust your budget, you should have created the best possible site you could have with your budget.