One of you main challenges will be how you make the solution externally accessible, i.e. how you exposed the application and data to the internet, for example via reverse proxy etc, and how you authenticate the users. Perhaps this is something your 3rd party can provide.
Once you have an idea regarding the infrastructure, you can consider application technologies, your application doesn't sound that complicated from what you've outlined above, however requirements such as mobile will influence the technologies you can use.
Many thanks,
Jason
