Slow speed smooth motion motor
The only way to go with the requirement of both significant torque and smooth motion is to use a stepper motor, gear box and microstepping driver. There are definitely servo's out there that can match performance, but they will definitely be a heck of a lot more expensive and more importantly, you will need to feed in an extremely precise voltage to regulate those. Here is an idea of the kind of calculation you need to perform:

Say you have a 1080p camera with a 45-degree angle of view. Let's ignore perspective distortion for a while and assume that each pixel perfectly divides its angle of view - i.e., each pixel horizontally sees 45 degrees / 1920 pixels = 0.023 degrees. Ideally, we want horizontal wobble to be less than one pixel (although in practice up to 3 pixels may be considered acceptable for videographic purposes). 

So, let's do microstepping. DRV8825 is a popular 32-microstepping driver that will be able to make your already very precise 0.9deg/step motor do 0.9/32=0.028 degree microsteps. Extremely close to our goal, but not completely there. Personally, I would definitely check this option out and try it before moving on to the next bit.

There is no (cheap) way to go smaller than 0.9 degrees, and no (cheap) way to go to more than 32 microsteps (although, microsteps aren't quite as accurate as full steps, you might get 0.033 degrees one time, and then 0.025 degrees on the next microstep). So this means you definitely have to go either gearbox or use e.g. v-groove drive. In the olden days of analogue media (LPs), this was often used to dampen the irregular motion of a driving motor and generate smoother motions on the thing that was actually driven. Also, this can give you an additional 1:3-1:5 gain you need to make those microsteps not matter anymore.