I did have my product first. I make table saw blade guards. I designed my machines to help out with production of these. I am still a full time plus some garage type home business. I would like to branch out in other areas, but really don't have the time. I always have had more orders than I could fill. Even with the help of CNC. The product pretty much sells itself through word of mouth.
I am always looking and thinking up other ideas for other products should the orders on these ever decline. The CNC's will certainly help to make different products in other markets possible.
About 3 years ago, I knew absolutely nothing about cnc or machining really. The Zone has helped me find answers for everything I wondered about in both cnc and manual milling and turning.
The satisfaction I got from designing and building my own machines and tackling the software learning curves was tremendous. I am still amazed by the machines and what they can do. Not just mine, but many of the machines you fine on here.
Ultimately though, I would say if you have a product in mind, good. Build a machine that will help you produce that. If you don't have one in mind, build one that can cut many different materials with only minimal swapping of parts. IE plasma cutter/ router. This would give you a broader range of possible product types.
I was pulling my plasma cutter around the other day. It is mounted on a trailer. I was coming back from Lowes with some material in it. A welding shop owner asked me wen I had stopped, just what that thing was sitting on the trailer? I told him it was a plasma cutter. He gave me his card and said he might like to get me to cut some things out for him from time to time. I said no problem. Give me a call. You just never know where that next job might come from.
Good luck with your venture.