That is hard to say and depends on a few different factors.
1. How far do you want to zoom in during editing?
2. What is the physical size of the print you are scanning?
Editing software does not care about "dpi" - it is meaningless really. We have to talk about PIXELS - what resolution is the scanned digital image?
Example - you are editing HD at 1920x1080 pixels. Therefore, any image you want to put into that program should be at least 1920x1080 pixels minimum, to fill the frame. Larger if you want to zoom in, right?
So maybe you have an 8x10 photo and it is Landscape format, so really 10x8 inches. Scanning at 150dpi would result in a file of 1500x1200 pixels. We can see that the result will NOT be large enough to fill the 1920x1080 "frame" we wish to put it in, so we need to scan at a higher dpi.
A 300dpi scan gives us 3000x2400. This would allow the editor to "zoom in" on the photo to some degree. You will need to experiment and see how far you can go before you run out of pixels and start "blowing up" the image, which is where you begin to lose quality. Maybe scan at 600dpi to be able to really zoom in on a face in the crowd!
Here's where it gets fun - what if the source is a wallet size, or 3x2 or 4x5? Then that same 300dpi scan comes up far short of the pixels needed to fill that HD frame, so the dpi of the scan needs to be bumped up accordingly. 600dpi? 1200dpi? More??
So that's why I said dpi means nothing to your editing software. A very small photo scanned at 300dpi would be useless for editing, while a large photo scanned at the same 300dpi would be more than enough to work with. So it's the pixels resulting from the scan that we are concerned with.
I'd try a few samples and figure out what resolution the image needs to be for you to zoom in to where you need to.
A tip - when adding pan/zoom motion to high-res stills, sometimes fine details will flicker during movement (teeth, eyeglass frames, etc). I've found that adding a small amount of Gaussian Blur in Photoshop totally fixes that! Try like just a 0.2 blur and that should do it. Loss of overall detail is not even noticeable, but end result motion looks 100% better. Just don't blur the original image, do a Save As to make a blurred copy to edit with.
When keyframing movement with Adobe Motion, you may want to add Ease Out/Ease In to starting and ending keyframes for smooth motion rather than abrupt, jerky stops and starts. Right-click the starting keyframe and choose Temporal Interpolation > Ease Out for example. The movement will ramp up smoothly, like a car leaving a stop sign. Same for coming to a smooth landing on your subject after a zoom, apply Ease In to final keyframe.
Do a search on this forum or the online for "Ken Burns Effect" to learn more about the whole process.
I've had better results shooting the photos with a 10MP, or higher, camera. Use small magnets or a good piece of glass from a frame shop to flatten out the picture. Much faster than scanning. Just make sure you have even lighting on the photo.