I can help you a little bit with some of these questions, but it would really help if you had a theme because then we could talk about what kind of plants and backgrounds you can have as an option. I personally made a background foam but I used the wrong substrate. you want to use a mixture of coco husk (the powdery kind) and a mixture of the coco husk with the big chunks to add texture and maybe a few rocks. you can attach them directly to the foam but I wouldn't recommend it since the foam is not entirely water proof. After the foam you want to apply black silicone but you have to get a specific kind. If you want something a little bit nicer then you can use Shield All which is easier to apply. Another sealant is pond sealant. again, a little more expensive than black silicone. Ok so you did the foam, applied the sealant and stuck the substrate onto the sealant before it dries.
now we talk plants which is something that interests me a bit more. A lot of people like to use tillansdia because you can mount it onto the background with a little bit of glue and all you need to do is let it grow. with orchids you run the hassle of constantly taking them out to fertilize, which is why I took mine out completely. I also notice a few plants that do well in terrariums such as certain types of moss, birds nest fern, bamboo, rabbit foots fern, croton, pothos, snake plant, and the list goes on. It really depends on the theme you are going for. Plus I would recommend getting a full spectrum light or a grow light to keep the plants healthy.
You can do a swampy paludarium, asiatic garden, woodland meadow with drift wood and moss. let your mind wander with this one and post pics as you progress with this project!