If you will pardon me a simple answer. Write it yourselves.
Who knows your product, service or organisation better than you do? Who is best fixed to answer any questions going about the operation of your company? Hopefully you!
Put as much thought into it as you need to and plan it any way that suits you but at the end of the day the best people to write up the textual content of your website are your people.
If your staff are too busy to write the content then we will happily provide the service for you. Creating content for websites on any subject is a perfectly reasonable thing to ask a webmaster to do. We will have to have a contact with whom details can be confirmed and we will need to see copies of any recent sales or marketing literature so that we can enhance your marketing in a joined up way rather than leaving you with a website that does not match the styke of your other marketing material.
There is no correct amount of text content for a site. The governing factor and number one rule is as follows. NO WAFFLE.
Once you are satisfied with your text and you have checked it twice for errors try reading it from a customers point of view and ask yourself some questions. Are there any relevant details missing? I the writing style consistent throughout and if not is there a valid reason for this? If you were looking for a product such as yours would you be interested in it?
Normally once these questions have been answered honestly a slight shift in writing style takes place to improve consistency, a few pertinent though small details are added and then you will begin to believe that you have come up with something readable.
Good luck!