Google App sheet Inventory App

We are looking to create an inventory app for the church using Google AppSheet. When purchasing any product, we will enter the following details: the date of purchase, reseller name, product code(autogenerated quote which will be used to scan the item) , internal item code ( to be used when scanning is not working), product description, purchase price, and quantity. During purchases, the user will scan the product code or manually enter the internal item code and it should show the item description if already exists and then the user will enter seller's name, quantity, and price. If it is a new item, they will have to enter the product description and other fields manually. When giving out an item, the staff will scan the product code or manually enter the internal code of the item. The item description should then appear, allowing the user to input the recipient's name, their prayer group, and the quantity. The inventory should automatically reduce when an item is given to a church member. Additionally, we should receive an alert if the item quantity falls below a certain level. We are not going to have less than 100 different items. For example, Mens Shirt Medium will have the same product code and internal item which ever seller we purchase it from. We want to keep it simple. We are looking for someone to create this and giving us a detailed document explaining the tables and fields so that we can make further modifications to the app internally.