You need to talk with all contractors you plan to use and be sure none are making extra work of the others. " framing not right for AC duct work so on. So to do it in phases will be harder and cost a little more maybe.
A good detailed plan comes first. Then walls and wiring. Hopefully, you will keep impetuous. I have seen many basements partially completed then forgotten. Good luck!
You need a Designer / Architect. They can help design a usable space for you and with today's computer programs you can do a walk though of the new space before it's even built. I as well as many other would be happy to help you in the design. See some of my designs here.3D view
Next framing then Electrical and HVAC.
You need to talk with all contractors you plan to use and be sure none are making extra work of the others. " framing not right for AC duct work so on. So to do it in phases will be harder and cost a little more maybe.
Blue prints