In order to set a member status variable you will need to set a session variable when the user logs in. You should set this variable to the value that is held in that column for the user. If you have Security Assist this would be done on the final page of the authenticate user server behavior. You would just give the variable a name and select the column that the value comes from.
When a user purchases a membership you just need to update the users record, you will set the member status column to a one. This can be done with a regular update server behavior.
To set the member discount in the shopping cart you can build a discount rule almost just like what you have described in the discounts tab of the cart object. To find out more info on this part you should visit the eCart support page. In the Solution Recipes and Archived Documentation are guides that discuss discounts and even a specific member discount guide. Here is a link to the eCart support page: