"Customise" product in checkout process....

In the Business edition, you can go in the menu Display->Custom fields and create custom fields of the table "item" of the type text.
They will appear on the product page so that the user can enter the information before adding the product to the cart.

1. There is no option for that, however, you can modify the file "show" of the view "product" to hide the item lines you don't need on some products. Here is a thread which talks about that: www.hikashop.com/en/support/forum/4-how-...uct-in-checkout.html
2. If you need javascript validation, what you can do is override the hikashopCheckChangeForm javascript function located in media/com_hikashop/js/hikashop.js in a js file of your template loaded AFTER that file and change the regex in it so that it checks for the format. If you need also to check for the fact that the user exists, you will have to do an AJAX request in that function to a new controller's task you would create which would check the email in the jos_users table.
3. I would indeed have proposed to remove the quantity box via the corresponding option of the configuration and only allow the user to add them one by one. But then, if you want to do all that, I would suggest an alternative:
Instead of creating custom item fields, you could create custom order fields. These are displayed during the checkout. Then, you could edit the file "fields" of the view "checkout" where you could add some code to only display as much fields as products in the cart. You could then do the check of the email in that view file more easily (or another one of the checkout, it depends how you setup your checkout workflow option of the configuration) than with the other solution.

1. That could be a possibility indeed. You could also have 20 fields created and just display as much fields as you need based on the quantity ordered...

2. It's indeed in the configuration, under the tab display with the option called "Display the quantity field on the product page". However, that has an effect on all the products. An alternative to that option would then be to edit the file "show" of the view "product" via the menu Display->Views and change that option on the fly for the product where you don't want to show it:

if($this->element->product_code=="my_product"){
$this->params->set('show_quantity_field', 2);
}