I will try and give some points on the things I have experience with. They are only my opinions and observations.
On backgrounds : In-tank backgrounds look impressive at first, but for me it gets boring after a time and limits what kind of aquascapes you can create. I like to change the colours and lights on the background depending on the mood of the aquascape.
On plants: If you are willing to unroot some plants from time to time you can also grow: Cryptocoryne pontederiifolia and petchii, Vallisneria, Sagitaria subulata, Marsilea quadrifolia and Echinodorus amazonicus. For stems you can try Bacopa sp. and some Hygrophylla sp. These are all plants I tried and successfully grew in low light conditions. In these conditions I find a trim once every 3 months is enough. Also try Bolbitis.
On light: Low CO2 and sunlight do not go well all the time. Have a plan to reduce the sunlight received by the tank when needed.
On substrate: There is no consensus
, you have to decide on your own. In my experience I had good luck with soil and bad luck with soil, so be prepared to tear down the aquarium if needed. I had good results in one aquarium after 6 months of trouble. Soil is very rich in nutrients so in a low light aquarium you only need a thin layer. I would only go with soil if I have a high proportion of Cryptocoryne, Echinodorus and Vallisneria. If you have anubias and java fern no need to muddy the waters (see what I did there). In the end I grew plants in 100% sand and in ADA aquasoil, just a matter of understanding what you have in the aquarium.
On filters: I am with team Eheim (not just because I am in Germany). They are expensive but you can buy one second hand. I think they are worth the money as they last and last. I had an Eheim classic for about 10 years and it still goes strong. At times I left it for 6 months without cleaning it (gross malpractice). I had a JBL which goes to half flow after 2 weeks, compared to Eheim after 2-3 months.
Also include in your expenses driftwood and rocks as they can cost quite a $. And inform yourself on tap-water parameters so you can select suitable livestock.