are you wanting an all in one tank, or are you willing to go with a rimless tank with a canister filter? I personally went for a truaqua tank with a ehiem canister hooked up to it, but you could go cheaper if your trying to stay inside a budget. or if you like the all in ones the best, those fluvals are pretty popular it seems
as far as plants go, I'm not a huge help. i have done nothing but low light plants in my tanks - crypts are my favorite. crypt parva can create a cool carpet if your willing to shell out some cash upfront to buy a lot, then give it plenty of time to grow in to look full
for dark substrates you could try aqua soil, eco complete, black sand, black blasting grit (totally safe - and as cheap as sand), fluval shrimp stratum, flourite black.. there are plenty other options out there I'm sure
if you are going to use actual soil, make sure to use an organic soil with no additives, sift all of the twigs and stuff out, and be sure to cap it with something. a lot of people use sand over top. a popular and safe soil that many people on here use is miracle grow organic choice potting mix.
If I remember correctly, aquarium salt is bad for most plants? idk for sure look into it. and shrimp don't have much of a bioload, its more a matter of whether or not your betta will want to eat them (some bettas demolish shrimp, some ignore them)
hope this helps