You won't regret getting the G540, even if they are a chunk of your budget initially. They do many things to make the process much smoother. If you have any questions about the wiring, etc, just ask.
I haven't built this design, although I do have the plans and am thinking about doing one for just metals. I did build my CNC machine (you can read about it here if you want) and used the G540 and am very happy with the results. I have cut wood, aluminum, and several plastics with very nice finishes. Don't forget to budget for software, depending on what you want to make with it.
I am also interested in a simpit. Haven't started, but probably do a modern fighter style since the instrumentation is easily done with flat panel displays. Always thought the CNC would not only be good at making the pieces fit well but also in making the skins look faceted and riveted, etc. Any pictures of your current sim on the net?
You also won't regret having a drill press, either. You might want to make table for it, unless it already came with a good fence. In any case, the work you've done so far looks great for never having owned a drill press before! One thing you just have to get comfortable with - building a CNC is an excuse to buy more tools!
You are also absolutely right to take your time - enjoy the build process! It won't be all that long before you will be wanting to build another!