I work with a lot new website projects, both from the standpoint of a brand new site and redesigns of existing sites. I will tell you from experience that both require a lot of work. To clarify this: they require work, not only from your website development company but also from you the company owner/marketing manager/sales, etc. I often hear the sound in our clients voices after we get to the stage of writing content for their sites that they really did not expect to put in hours and hours of work making sure that they have enough/quality content that will satisfy the search engines as well as the users.
This does not have to be an overwhelming task though, if you are prepared from the start. I know from an SEO perspective that if you plan ahead on who will be taking on each task (our project management team lays this list out very well) in the process your success and stress level will be greatly reduced.
Starting with deciding on an initial keyword phrase list that covers all that your website will be about. Next deciding on which phrases you will focus on. After that, it is on to writing content that matches those keyword phrases to a highly focused page within your navigation. The writing is perhaps the hardest part for most businesses as they are just that, experts in their industry not copywriters. This is where I would suggest using the services that most web development companies will offer and that is copy writing services. They will interview you and your team and be sure to get your thoughts and facts expressed clearly on the page, and if they are really good they will incorporate SEO tactics while doing so.
I guess the bottom line here is that creating/updating your new website will not be easy, but with the proper guidance and preparation it should go smoothly and have the outcome you were looking for.