Alex here, from the Shopify Support team.
Oberlo is completely free to use, as long as you have under 500 products imported and under 50 orders per month fulfilled through the app! To import more than 500 products or to process over 50 orders per month, you can sign up for their Basic plan at $29.90/monthly.
You can see a more detailed breakdown of what the plans offer here.
Feel free to follow up here with any further questions!
This is Max from the Shopify Guru Team.
You can definitely import products through other apps or suppliers using Shopify as your shop.
I hope this helps you get started researching the service that's best for you.