You can access your purchase receipts via the customer hub.
Yes, I recommend that you do. You are not required to, though. You can simply add your website URL or logo and sell the end products as your own.
Yes and No.
Rarely do I update the products. I have a lot of them and continue to release often, so there's little time to go back through older products.
I hope you have a plan before you buy product, but here are some recommendations: