Hi Sesalie,
Every experienced shoppers will tell you to sign up for every MSC that is on the official list at the bottom of this page. Each company has clients in different areas, and they change all the time, so being signed up with all of them is the only way to know when opportunities come up in your area.
Sounds like you have a good game plan starting out only accepting shops that don't require any capital outlay.
There are some users on this forum that claim to make a full time livable income mystery shopping, I have no reason to doubt them, so I imagine it is possible. Personally, it is a secondary income, and a chance to take my family out for activities and dining from time to time.
Best of luck to you, and welcome to the forum.
proudly shopping in the D.