How can I edit the Add Product page?

I want to make a few changes to the Add product page.

1) The field where you fill in the price should be pounds, not dollars, and it should be only one field (no shipping, tax etc)

2) Each product should be unique i.e. the quantity is always 1 and when a product is bought it disappears of the market

3) I am using a plugin called Multisite Global Search but it doesn’t return results for the Product pages. Can I edit the plugin in some way?

It will be nice if you give me some direction e.g. which file should I edit for 1) and where should I look for 2).

Thank you very muchfor your support.