Hiring a web design agency means you've very little to worry about. There will be a cordial client service team to take care of your needs on one side and experienced professionals to work on your project on the other.
You can expect a level of professionalism in every aspect, right from the first meeting through to project delivery and afterwards. More importantly, your website will be optimised for Google as well as for other search engines' crawling and indexing. In an agency, there will be many resources deployed for project completion. So even if one or two people go on leave, you’ll get your work completed on time, as promised.
Above all, agencies will always be technologically updated, so you can have your site built on the latest and best platforms.