Honestly, 40 accounts seems like a little much to jump right into. I doubt if it can really be done part-time with good service to your customers. Starting my own company, I want my customers to feel a nd know that they are important to me. I am choosing to start on a smaller, profitable, customer base that can grow with my expertise.
Also, $400-500/wk. seems a little low. I know that YOU are just starting out, but you are talking about an existing business that should be profitable. I would be questioning the quality of accounts. Don't ever take someone's word for it when it comes to buying their accounts. Even if they are a friend. Business is business. Ask for proof of expenses/gross income. And I would want to meet all of my potential clients BEFORE I buy anything.