Order (limited amount) of tickets (Products) with Webform

Hi,
I have been looking extensively but not found a solution.

Here's what I want to accomplish:

Create a simple form with fields:

Name
Ticket A
Ticket B
Ticket C
E-Mail
Submit

The site maintainer should be able to set the number of tickets available for a specific show (drop down menu select 1, 2, 3, etc..). The user can order tickets with the web form, choosing the number of available tickets s/he wants. On hitting Submit, two order confirmations go out to the site maintainer and customer who just ordered the tickets. There should be an automatic update of the number of tickets available so that the next user can see how many tickets per category are still available and choose accordingly.

For me the tricky part is to set up select boxes which dynamically change their contents. Say there is a total amount of 80 tickets for category Ticket A available and if a customer orders 20 tickets using the form, the next customer sees that there are 60 tickets available. If all tickets of a category are gone, a message appears and the select box disappears thus making it impossible to order any more.
I do not need any payment solution, the tickets can only be reserved and must be paid at the counter.

Can anyone help or post a link to a tutorial?

Thanks
Chris