So long term plan, in my mind is start off as a small job shop while working at my day job to get cash flow. Reinvest every dollar into the business I can. The 600' is where I want to start because there is no overhead, but I completely understand that unless thing go poorly for me, I will outgrow it very quickly. As far as a market to target I'm not really sure. I live near St. Louis so there is a lot of aerospace and medical work in the area, but I don't know how much of it would require certification.
Now it seems I have significantly overestimated the cost of starting a one-man shop. Many people in this thread seem to think that I could start up for 20k -30k but I don't really see how that's possible. Maybe I just need to adjust some of my expectations of what is necessary but even then, the used mills I've seen recently, like a used Haas VF 2 SS, sell for near 40k.
The dream is a moderate sized shop specializing in multi-axis and additive manufacturing as well as a design consultation firm that helps with DfAM conversions of assemblies. I know that it will be a lot of work and that the dream might change between now and then but that is the target I'm shooting for now.