Thinking more about the idea of getting a local builder to do it for you. Other than what has already been said about material costs, they already have the know-how, experience and the tools to do the job. What is more, they can do you proper electrics, with an independent fuse/breaker box in the workshop, and power running into the workshop from underground. That would be much better than the the extension power cord running overhead between the house and the shed, which can be a nuisance as well as a hazard.