First, promotions and PPC is not available for Individual Sellers. The Selling plan comparison help page highlights the differences.
From a fee perspective, an Individual Seller is assessed a $0.99 per unit fee when you sell. A Professional Seller waives this and replaces it with a fixed $39.99 monthly fee. So, simply focusing on fees, if you plan or are going to sell more than 40 items a month, the per item fee that is saved will pay for the fixed monthly fee. All units sold above 40 in a month results in an additional $0.99 in savings to you. All other fees are the same, regardless of selling plan chosen.
In addition, many categories require a seller to be a Professional seller to apply. A professional seller also gets access to business reports so you can see the page activities generated on products you sell. When you look at a product page, the ‘Add to Cart’ seller on the page requires a Professional Seller account.
You can use feeds and upload sheets to manage/add/remove your inventory if a Professional Seller. Individual sellers have to do them one-by-one. A Professional Seller can set their own shipping rates and offer free shipping. Individual sellers are not able to modify the shipping rates for sales.
If you have multiple employees that you want to be able to administrate your account, you will need a Professional account. If you want to use third party shipping services like stamps.com, shipstation, etc. you will need a Professional account to link to your seller account.
Take a moment to review the link above and the links contained on that page to get more details. If you have specific questions please post an update.