I understand the feeling of being overwhelmed at this point. My suggestion for getting a handle on the complexity would be to look closely at several other peoples' successful lathe conversions, make a decision about which approach appears to be within your capabilities to duplicate, and then use that one as your pattern. There is no single way to do these conversions, rather there are many approaches that have been successful.
I have a smaller, 7x10 mini lathe that I would like to convert to CNC, however I have also considered the possibility of getting something larger (such as a Grizzly G0602 which is a 10x22 lathe). There are several kits available for the 7x10 (or 7x12 or 7x14) mini lathes, and there are a number of DIY conversions that have been carefully documented as well. By way of contrast, so far it appears that all of the G0602 conversions that I've seen have been DIY ones. The size of your lathe puts it squarely in the middle of that territory.
Can you narrow down the types of operations that you need to automate? For example, for the types of parts that I want to produce on my lathe, I need to do turning and facing and parting operations, and perhaps also center drilling operations, however I have no need or plans to use my lathe for threading of any sort. So for me, that means that I don't need to be concerned about generating or detecting a spindle rotation index signal, or about precisely controlling the motion of the spindle. I can probably get away with manually controlling the spindle rpm to begin with. What operations must your lathe absolutely automate? Can you simplify things to start out by not automating every facet of the lathe's operation?
I don't know of any off-the-shelf automated tool changers for mini lathes - a lot of folks here have built their own turret-style tool changers, based off of a common design. Some people have considered making their own gang tooling attachments.
You can probably hold off from making decisions about the details of the stepper motor controller and the PC until later in the process.
And so on...