Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
You could keep most of the small to mid sized plants (appropriate for a 15" tall tank) in as little as 1" of dirt. I think I would try for 1" at the front, then use rocks and driftwood to make a retaining wall, and see if you could get it 2" deep in the back. Some of the larger plants will do better with a bit more dirt.
But 1" is plenty for the smaller plants that you could use in the front.
If you want to split the substrate between sand and dirt use a solid retaining wall, such as rock or driftwood with expanding foam filler (black, used for ponds) to really keep the sand and soil separate.
No much you can do about saving weight with the set up you are envisioning.
Here is a way, though not pretty:
Bare bottom, plants in pots.
Much easier to move when the time comes.