What i think for sure is this:
Do not buy anything more until you have definitely finished your drawing and have it all clear 100%. That includes rails, screws, motors, board, etc. It will be just throwing money in the bin. And yes, we have done it all.

One more thing to note: Everyone here thinks he is a top machine designer. Nothing wrong with that. And in reality here we could see most of the best build ever worldwide. But what i learned from experience is that even if you overthink your machine, you always mess something, even small. Once you make the machine though, use the machine 1 year 8h per day, maybe make another one later, revise a couple of times the design customising it for clients, then this machine becomes better and better.
What i am saying is: assume you will make errors, if someone has made a nice working machine and you like the design, ask, listen and copy shamesly. Do not over think and over invent. look for good working designs, easy to make at home.