The foam usually used for terrarium background is polyurethane foam (great stuff).
for painting, the better choice is acrylic paint and for sealer i use tiling sealant. But these materials are not safe and you have to cover them with epoyx resin for example.
For orchids it depend on what species you buy, but some small orchids fits onto this kind on set up.
A lot of people use coconut fiber but i hate this material, everyone his taste

plants which grows in your woods are not adapted at all, you must buy some broms or rainforest plants on a specialized website. Avoid taking plants, ... in the nature, we never know...And for the soil you can, for exemple, use exo terra plantation soil, it is fully safe but don't forget, brom's musn't to be planted in the soil or they will rot. it is a bit difficult to create a good rainforest set up when we are an absolute beginner.

---------- Post added at 09:30 PM ---------- Previous post was at 09:22 PM ----------
And i forgot, for the light, it depends on the size of the tank (lenght, height, ...). And you musn't forget that, at the origine, an aquarium tank is not adapted for rainforest set up and you must adapt it with fans, or holes with stainless steel with wire mesh... for getting good air circulation otherwise you will have too many mist on glass and maybe some mould