Order Validation AddOn

[Polythene Pam]

I would like to create an order validation add-on. 

The main features are:

• Right before submitting the actual order on the prestashop, I would like to validate the stock of all the products on the order using an external webservice. 
• This validation might also return different values for the delivery process due to remote service specific calculations.
• If the order external validation process is successful, the order is placed immediately and if not, there should be a message on the order submission page and the order should be submitted or not, according to user selection.

Where should I start? I've already read addon documentation and now I'm looking at this addon wizard:

https://validator.prestashop.com/generator#

My questions using the generator are: 

• What Module type should I use? A payment module? A generic module?
• What could it be selected on the Module tab? Checkout ? Payments and Gateways?
• On the last step, which Hook should I choose? (in what hook should I place my module).

Any help, ideias, etc, would be appreciated.

Thanks.