Magento 2 customization

We have a magento 2 instalation with the css gridview extension. The products in the site are based on variants. All the variants are displayed in the product gridview extension. The tasks that we need to perform, All products and categories are loaded in the Magento 2 - Premium theme instalation (theme provided) - Payment getaway extension (Provided) Customization in product gridview extension - in the table, we show stock of the variant. We need to replace this by ("inmediate delivery" + stock) when stock > 0 or the string "7 days delivery" when stock is = to 0. Cart customization. As some products we offer we have in stock, these can be delivered next day but when is not in stock, it takes 7 days.... so we need to let the customer choose to have 2 deliveries if he purchase products in stock and not in stock, but prefers to have an early delivery with the goods in stock. We need someone available to start a.s.a.p