Thanks so much for the replies!
Indeed, it makes a lot of sense to try it out first.
So here's what I will do: First, today I will take pictures of another geagraphical feature (a smaller landscape) and try to import them in PS. Then, provided the results are encouraging, I will plan my visit to the valley with the help of fpbv (thanks for the offer!), considering the lens, height, etc.
My alternatives at this point are to hire a lidar survey (~ $30,000 USD) or a field visit with a topographer (3 ~4 weeks to take physical measurements with a handheld GPS). So I would much rather make this approach work!
10 cm resolution would be overkill, but if the data is there, I'll certainly use it. 1m should be more than adequate. Do you think that 5,000,000 polygons will be enough to map an area of 2,000,000 pixels, considering that there are large variations in the height of the features on the land (peaks more than 1,000 meters higher than the center valley) ?