Force COUNTRY, on SHIPMENT address, according to CATEGORY

[AhoyIT]

Hello all,

I'd like to create a Plugin, that would do the following:

A. Create admin setup page, with a list of COUNTRIES and a selectable COMBOX with ALL existing CATEGORIES. Enable linking a country to a category.
B. When customer, opens SHIPMENT address page, COUNTRY field will by be set to a pre-defined value (As per admin side, above) - According to selected product category.

For exapmle, if on admin side - USA was linked to CATEGORY 4 ... Then, on front-end - After adding an item from category 4, When customer opens Shipment address page, he'll see USA selected by default.

My q's is - What type of PLUGIN do i need to use and where\how to i trigger it?

[AhoyIT]

BTW,

Event trigger seems to be "editaddresscartST" ... Couldn't find a trigger for it - So how do i go about creating a plug that would?