Getting started to work on my 1974 Dart Sport. I got questions

The rabbit hole worries are real. Me personally, I would give the auto trans a shot for now and see if you can keep the car on the road for as long as possible while you get more skilled mechanically. Once most everything else is done and you want to tackle that 4spd conversion, then go for it. Trying it right away could get you in over your head and drop your desire to work on the project down to a point where you won't want to touch it for long periods of time and when you do, it could seem like a never ending daunting process.

The 904 and 727 autos are pretty dang bullet proof for the most part. The fact that the car was raced could mean that there is extra wear and tear on it but the first things I would check would be the fluid level, the shift linkage and the kickdown linkage. No need to throw a bunch of money at it, just add fluid if it's necessary and checking the linkages costs nothing but time. Run some searches on here in the transmission forum to get more familiar with how to do it.