With regards to substrate, the common thinking is that Cory's prefer sand and gravel will wear down their barbels. My 200 litre tank has roughly 65% gravel and 35% sand and they seem to have no preference and dont seem to have issues with worn barbels. With regards to plants, as long as you arent doing any high demand plants then either as well. Non-rooting plants like anubias, java fern etc dont even need a substrate. I used a layer of tropica plant care substrate beneath my gravel/sand layer and root tabs after a year when the plants looked to be lacking some nutrients. I would say just go with your own personal aesthetic preference, but if you go gravel try and get something nice and smooth.
I would also do your hardscape and planting before you fill your tank. It will be easier, plants try and float away to the surface while you are trying to plant them.
This is the first in a series of a low tech planted tank project. Really got me started on planted tanks. Have a watch through the whole series, it covers a lot good pointers.