All of my bags have been made from dollar store placemats. When I am at Joanne's I always look thru the remnants for pretty fabrics to make the linings. For the handles, I have used fabric, accessory chains, belts or ribbon. I love making my bags and I get so many compliments on them. Try one. I am sure you will do great! Show us when you do you first one!
I have never purchased a bag and embelished it. I start from scratch and make the bags.I just think that gives me more options to be creative. But that is my way. You can do it any way you like. It might be easier to start with a purchased bag that you really like and embelish it. Then when you feel more comfortable doing it, start from scratch. Just one suggestion, if you start from scratch, do your embroidery before you put the bag together. That way is easier and your stiches will be hidden by the lining on the inside. I say "go for it". *4U
What I've seen so far in the projects are bags made by the person herself, bags made from placemats, bought bags. So it's up to you. Good luck.
You can do it either way. You may have to open a seam in some purchased ones to get it in the hoop.