i keep thinking about doing a new shop area. I feel the important things to remember after concrete as it has been talked about are things like accessibility to move equipment in so rollup door or double door will allow a forklift in. A layout that will allow wall to isolate wood working area as dust can be a real issue even with dust collection. Height so it gives good overhead but still easy to heat and cool. Insulation for noise and heating and cooling such as Minisplit. Electric and water as needed.
The big thing though if size wont be huge I feel a longer shop allows separation without a lot of wasted dead space. 18'x 24' to 36' would fit a lot of equipment and allow for splitting your work areas. minimum. If you can go bigger, do it. I hope you post what you have decided. sorry if I missed it.
On another note I would buy the correct mill and lathe for the work you plan on evolving into. I have access to a Bridgeport and decent lathe so use them a lot but I own a mill/drill and small lathe. They will do a lot of work but I am always think about upgrading them to what I really want as they have many limitations.
