Well for starters, you'll want to keep your silver dollar with a school of other silver dollars, each one of those will get to six inches so you've got quite a lot of fish right there. If you end up keeping the silver dollar with some friends, that kind of kills the small to medium fish idea, though you could still keep some smaller fish in there. Check out gouramis if you're looking for a loner sort of community fish, either keep a single male or one male with two females. You've got a lot of options for bottom feeders, check out some
loaches,
corydoras, smaller plecos, other catfish or all of the above. Most loaches and all corydoras both like to be kept in groups of at least 5. If you plan on breeding anything, you'll want to keep a species only tank and figure out the breeding requirements of your particular species, if they'll breed in aquaria at all. You'll also want space to house all of the fry, I really wouldn't recommend it.
Sand, gravel and small pebbles will work fine for substrate, most bottom dwellers will appreciate a sandy substrate and I find it to be easier to clean than coarser substrates. A lot of hardy plants will do fine in these substrates but if you really want to get into a heavily planted tank, you may want to look into special substrates. Sounds like you've never done live plants before, I'd suggest starting with a few easy plants like java fern, java moss, anacharis, anubias and cryptocorynes. They all have their little quirks, but will do fine in most conditions. Check out the
planted section for more info on planted tanks.
I'd suggest looking at a canister filter for a tank of your size, it will have space inside for biological filtration media (often ceramic or plastic "bio-balls"), mechanical filtration media (usually sponges or filter floss, can also double as biological) and chemical filtration media (as needed). You'll also probably want to look at doing a fishless cycle or borrowing filter media from an old tank to clone your new one.