Adding custom fields to checkout products not in webstore

We are not able to upload all parts that customers ask for to our webstore. As this is a B2B store with already existing customers, we have to be able to offer them to buy all the parts that are in their documentation from our webstore even if we don't have the product in the store. The solution for this will be adding custom fields in the cart. Two customs fields, one for article number and one for product description. The should be at least two rows with these column, it would be great if it would be possible for the customer to add new rows with some ajax functionality or something. When the order is complete, the information typed in in the custom fields should appear on the order confirmation, order confirmation email, backend, in the same way a normal product would appear after an order. We will not process the orders in Magento, since we need to manually insert the order confirmation in our ERP system. So there is no need for us to change the order after it's placed. But if this is possible through a module or at a lower cost, it's a nice to have feature. This will work almost work like a quote functionality, but our customers will order both products already in the webstore and use this custom fields. Do you know any extension that could handle this or does it need to be manually coded? How many hours would a project like this take? Please let me know if there is any questions. Thanks!