If I understand well, you want to change the customer group when a customer bought for a minimum amount.
When a customer has the amount, he must be switched into another group, for example "Gold" and then have access to another shop.
I think that you can do a plugin based on the function "onAfterOrderCreate()" which will execute a SQL query to have all the products bought by this customer, then check the amount, and depending on the amount change the customer group.
In the shop, you can restrict menus, modules, categories and products to a group of customers, that will be the way you set the different shops.