It would help you get some replies if you can post some photos of your machine here. They would answer a lot of our initial questions about your conversion effort and serve as references for debate as you make your build plans.
Side loads on three router bits will likely require higher torque steppers than what a lot of us hobby CNC builders are using. If your routers don't have any significant flexing around mid travel (are they on a moving gantry?) then you just need enough torque to cut at the feed rates and cut depths that are acceptable to you. Take a look at servo drive systems and decide if that is the way you need to go.
Don't know about the dual motors but I personally would use a larger motor and simplify the drive mechanism as much as possible to avoid any backlash. Use toothed belts instead of gears if reduction is needed.
If you only need to cut from top, bottom, and two sides you can use a purely mechanical indexer for the 4th axis. If you want the stocks to rotate while being profiled then you need a drive motor. The three stocks can be coupled together with toothed belts and pulleys. I've seen non-CNC duplicators that used bicycle sprockets and chain with a wood disk with indexing holes and wood pegs that worked well enough for the old production machines.
Tool changers are available but the posts I have seen about them generally say that there are run-out issues with them unless you pay the money for the absolute best ones. It doesn't take much run-out to be useless at 20,000 rpm or so.
Roton ballscrews work well enough for many applications but it's another case of if you need the accuracy for metal machining you should pay the price for the higher precision ballcrews. Do wood gunstocks really need high precision ballscrews? Probably not, but precision ballscrews will probably hold their precision longer.