Starter Kit - Painted Action Figures

I have a handicapped son, Down's Syndrome, who is interested in action
figures.  He is not looking to have mock battles or stage contests.  He will
be able to sort out the figures, arrange them by type - color - pose -
armament etc.  He will be able to move them around but they must be free
standing or have a base.  I would like him to have a table or work surface
with some simulation on the surface, such as contours, grass, trees etc.

My questions are the following:
1. Not sure he will like this so would like to start simple.
2. Figures need to be painted - he does not have that skill.
3. What scale?
4. Is there such a kit on the market or does one just buy pieces?  (I am a
model railroader and would never think of buying a starter kit - for many
reasons)
5. What era, style or situation he works with is not important.
6. What other considerations do I need to give to this?