You have asked a lot of questions in one go
Volts times amps = watts, which is a measure of how much work the motor can do, so:
36v * 2.3amp = 82.8 watts
24v * 9amp = 216 watts
So your 24 volt motors are capable of producing the most power, if you have a power supply that can hand it over.
But, your coarse pitch screw and high rpm motors are kind of working against one another, because I doubt you want to move at 3750" per min in Rapid This means a fairly substantial gearing down to achieve the proper range of output. But, that is not necessarily a bad thing. If you use timing belts for reduction, you can always change the ratio after you have the machine built.
You need to check the maximum encoder counts your controller will be able to process, as this may influence where you mount the encoders, as well as how many lines of resolution they would need.
An encoder mounted directly on a high rpm motor will generate a lot of pulses, and may overload your controller's feedback capacity, limiting your upper rapid speed unexpectedly.
However, mounting your encoder directly on the screws is considered more accurate. This will allow a higher resolution encoder to be used. Higher res encoders cost more, of course.
Whichever way you mount it, the servo PID settings can be adjusted to accomodate the system. Servos operate according to the amount of error between the present position and the commanded position ( with an upper limit of course). The output voltage is proportional to this error, so the controller really could care less what the servo/ballscrew gear ratio, or what the encoder ratio is. When the encoder feedback shows that the machine is getting near the commanded position, the output voltage is cut back smoothly and quickly to zero, as the commanded position is reached. The PID parameters adjust the way the servo motor settles into position, controlling overshoot, undershoot and oscillation.
What this boils down to is the accel/decel ramps that you will set up are going to be the ultimate limitation for your chosen drive amps. Acceleration is typically when the controller commands the amp to output the maximum voltage. A sharp current spike here will trip out the overload protection on your drives. The actual inertia and friction of your machine slides produces the limitation, and I don't know exactly how you would know in advance what you will be able to achieve in real life rapid speeds.