Modification on modified webshop

We have a modified webshop, with a fix discount of 4% in the checkout. This discount is hard coded in the [login to view URL] We want this discount according to the customer. 1. customer table needs to have a discount column. 2. In the administrator which is modified, the customer discount must be editable. The is a customers edit screen in de administrator. 3. After login the % of the discount must be shown in a box. 4. In the shopping cart the discount must be calculated according to the customer. This is already in the system, only now it is fixed. This is rather urgent, I'm looking for someone who can start right away.