Welcome back and congrats on the new edition to the family!!
I thought everyone had lost interest in the Pi-rotate project, so I kinda shelved it.
I do need to work on mrotate for Pi again, a few loose ends I need to fasten down, so I will try to get the project back out and dust it off.
May be a few days though cause life is getting in my way.
About the motor controller: Yes you will need a device that can handle the large power (compared to a Pi output) that it will take to run the motor. This device is normally called a drive.
There are several drives available on the market, and if you have time, patience, and the tools you can build it yourself.
However there are so many cheap options out there it may be best just to find what you need and buy it ready made, unless like me, your first hobby was electronics and you enjoy "Building your own".
What type drive do you need? This depends entirely on what type motor you choose.
So you need to choose your motor first.
What type motor? That can be a tough question, or it can be very simple.
You need to figure out how much power you need to turn the monitor.
How heavy is your monitor? What type of bearings or pivot do you plan to use to allow it to turn? How fast do you want it to turn? (Too fast can be dangerous and too slow is frustrating.)
You need decide what type power supply you need for the motor. The power supply must closely match the requirements of the motor, and it must supply the current needed for the motor.
Seems the most common motors used for this purpose, IMHO, are DC brush types, 12 volts DC.
How big the motor is goes back to the second line of this paragraph. Bigger the motor, more work it can do.
But bigger the motor, more power required to run it, more space consumed in your cabinet, and probably more noise when running.
The speed of the motor is important too. Although you can slow a DC brush motor down by decreasing the voltage going to it, you want to make sure it can go as slow as you want it.
If the motor is still too fast (you only need the monitor to turn 90 degrees), you will have to reduce the speed by using gears or pulleys and belts.
It can get complicated mechanical wise. But you can buy DC motors with gearboxes attached that does most of that work for you.
The windshield wiper motor is common because it needs 12 VDC, which can be supplied by an old computer power supply. It has a built on gearbox, which means the output is pretty slow, and it is a strong motor.
I used one on my first rotate project that turns a 19" CRT monitor.
But it is loud when it runs.
Other motors that people have used on here are the tiny motors (forget the name), steppers, and servos.
Steppers and servos require special drives and most likely digital interface with the computer, which Mrotate for Pi will not do.
I used a actuator (screw) motor on my experimental Pi thing for this project.
So I guess you first need to concentrate on how you are actually going to mechanically turn the monitor.
Build the frame for that, see how much force you think will take to move it, then decide on the motor and how you will couple the motor to the monitor, then pick out the motor, the power supply, and the drive. In that order. (I would keep the power supply for the motor separate from the power supply for the Pi.)
I know this seems a lot to chew, but it's not really that bad, I do tend to ramble a lot in my old age LOL.
Just first concentrate on how you are going to perform the act of physically turning the monitor.
We are here to help, several people have done this and I havent heard anyone say they regret it.