1.) Rona, Reno-Depot, Home Depot, look for regulat top soil, no chemical additives?
2.) 1 inch dirt, 3/4 - 1 inch sand
3.) You need to keep some extra gravel to cover roots that don't play fair when you plant.
4.) Start with what you have. Low tech, lots of plants, all my tanks are what one would say "under filtered". Plants play their role.
5.) If you are on a budget, turn it off. I don't use CO2. It's probably easier to turn it on later than turn it off... Also you will get a better feel as to how low tech/dirt tanks behave.
6.) Lots of contraction on this. Monitor ammonia, nitrites and nitrates. For as long as you have nitrites, it's not good for fishes. And nitrites don't show until ammonia builds up. On the other hand I read that it can be ok under some circumstances to add fishes right away. I guess that means lots and lots of plants to absorb ammonia before it turns into nitrites...
7.) Not really, but it's not worse either.
8.) I have 3 dirt tanks, one gravel tank between the living room and kitchen. No smamp smell. Been running for over a year. All tanks but the gravel one never had water change either.
It's a lot of fun. I have a fifth tank waiting for a stand