I am not talking vehicles, overhead, etc. Just the license and insurance to be able to go out and do a job?
Others above have mentioned the licensing, insurance, continuing ed costs, and tools. I know you said your not interested in vehicle or overhead costs, but I don't see how you can do work without those costs.
A vehicle is absolutely required. You're not planning on walking to these jobs pushing a wheelbarrow full of tools and materials are you? Even if you use your personal vehicle, there will be additional operating costs such as gas, oil, tires, and wear and tear that will cause maintenance. This assumes your personal vehicle is big enough for ladders and 10 foot sticks of pipe. Planning on using a trailer for those? Trailers aren't free either. Even if you already own one you are wearing it out using it. And pulling a trailer increases wear and tear on your car and costs extra gas. If you really are planning on pushing a wheelbarrow, keep in mind--wheelbarrows cost money.
How are you going to avoid overhead? If you buy some material, will you or your bookkeeper enter the data into Quickbooks? That's overhead. Doesn't matter if you or your wife are doing it. No overhead work should be done for free. You must pay yourself or the person doing it. You wouldn't let an employer make you work for free. Why do it to yourself? Looking at jobs? Overhead. Estimating and bidding? Overhead. Opening a bank account, paying bills, hiring, firing, payroll? Overhead. Taking service vehicle to repair shop, writing up advertisements, tax filings, trips to update business license, working with a web designer, buying office supplies; the list is endless.