User Interface for B2B ordering system

To create a user friendly UI that lets our customers place B2B orders. It is important that you have strong design and UI experience. This new system has to be super easy to use, in order to be adopted by our clients (who currently email us, and then we manually input orders into our backoffice database). Objectives 1. Create order 2. Lookup and view past orders 3. Integration with back office database Features 1. Customer creation (by Admin only) 2. User creation (by customer) 3. Look up item via search box 4. Look up item via browsing categories 5. Order by item numbers 6. Inventory display rules ‘in stock’ ‘out of stock’ 7. One time discount code (off total invoiced amount) 8. Update last sold price into back office database (unique to customer and SKU) 9. Update inventory into back office database 10. Feed confirmed orders into back office database automatically 11. Payment with PayPal or credit cards 12. Look up order (tracking #) Please indicate your experience with custom software development like this, and suggest some open source/off the shelf software that we can use. We value your input and will need help with implementation in both scenarios. Thanks!