Edit: I think I found your camera, PowerShot SX70 HS. Apparently, it allows remote control of the zoom and is equipped with a huge 65x zoom.
The best camera for your situation, because you do not absolutely need a large sensor and can replace a large sensor with a long exposure, it is a compact camera. Unfortunately, Canon has discontinued the programmatic API control of its PowerShot cameras, as you've noticed. Some old PowerShot and IXUS cameras can be controlled remotely: http://www.gphoto.org/doc/remote/ … but you need to find a used camera and its Mpix rating will be low. Therefore, if you can not find any compact cameras that can be controlled by remote control, your only option is a digital SLR. Compact cameras have the advantage that the zoom occurs with an electric motor, which means that in some cases it is possible to control the zoom programmatically.
The EOS DSLR offer an advantage: a larger sensor (which does not need) and a big disadvantage: the zoom is not controlled by an electric motor, so the zoom can only be done manually. So, if you decide to go with the EOS digital SLR camera, you need at least two, maybe even three (and a small amount of digital clipping) in practice.
The Canon Camera API is compatible with EOS 4000D, which is usually shipped with an 18-55mm zoom lens without stabilized image. You could buy two, for $ 300- $ 350 per piece, and set your zooms in different values. 4000D is 18 Mpix, but if you need to crop the images, it will be reduced. Keep in mind that the optical quality of 4000D and the lens sent is quite good and the noise is low, so the 18 Mpix is 18 real megapixels, not a useless advertising value.
If you want to go a little more expensive, the EOS 2000D is compatible with 24 megapixels and is usually shipped with a stabilized lens for the image. Are you willing to pay for the additional 6 megapixels, only you know it? Image stabilization in your case is an unnecessary feature.
The 2000D and the 4000D can also be easily placed on a tripod using a standard screw mount.
Your need for zoom is 6x. If you set one lens at 2.45x the other, in the worst case you need a digital cutout of 2.45x in both directions. 18 Mpix therefore, in the worst case, it would be 3 Mpix (18 / 2.45 ^ 2 = about 3). But the images at 3 Mpix would be extremely sharp, maybe that could be enough for you? 24 Mpix would be in the worst case 4 Mpix.
If you buy three digital SLRs, and establish:
- One to 18mm
- One to 31.5mm
- One to 55mm
In the worst case, you need a 1.96x cut, which means that 18 Mpix becomes 4.7 Mpix and 24 Mpix becomes 6.2 Mpix. If you can move the 55mm camera a little closer,
- One to 18mm
- One at 32.7mm
- From one to 55mm and get a little closer.
… in the worst case you need a 1.82x cutout, which means that 18 Mpix becomes 5.4 Mpix and 24 Mpix becomes 7.2 Mpix.
My advice? If you plan to use the cameras also for other purposes in the future and not just for this project, buy a 2000D with a stabilized lens for the image and learn how much the megapixels mean. Then, you can decide which other / two cameras to buy, 2000D or 4000D. Maybe even 3 Mpix would be enough for you if the images are really sharp?