Depending on the number of users and how you publish to the web, you may need just FileMaker, FileMaker and FileMaker server (Custom web publishing only), FileMaker and FileMaker Server Advanced (Instant Web Publishing).
You'll want to read up here on the product info pages to learn more about the capabilities of each. Also keep in mind that if you contract with a company that can publish the database to the web for you, all you need is FileMaker.so that you can develop and test the database before deploying it via the hosting services server.
On paper, FileMaker can do all you describe, but the devil is truly in the details. I suggest downloading a trial copy of FileMaker so that you can get a good hands on idea of not only what it can do, but how well your particular skills and experience equip you to get this done. It will also be a very good idea to read the PDF document on web publishing that downloads with FileMaker and which can also be found in their downloads section. (Drill down to the documents area.)
If you do decide to invest in this application, plan on investing some time using training tutorials, books and other resources to learn more about database design and how to use FileMaker. Also, make sure to get a single copy of FileMaker Advanced. Its development tools will save you hours of work and is well worth the added $$$.
Apologies for the delay in responding - I've been on annual leave.
Thanks very much for the info - that's exactly what I was looking for. I will purchase a copy of Filemaker Pro Advanced and let you know how I get on with it. :0)