Welcome to Tropical Fish Keeping forum. And welcome back to this wonderful hobby.
You have many options for a 38g bowfront. The first thing to decide is what fish will be suited to your source water (presumably tap), as it is much easier and safer to select fish suited to the water than it is to get into adjusting water parameters which can certainly be done but often with considerable effort and expense, depending which way one is adjusting (making soft water harder, or the reverse).
You can ascertain the GH (general hardness) and KH (carbonate hardness, or Alkalinity) from the municipal supply folks, probably on their website. They should have the pH too. Once we know these numbers, we can start narrowing down options.
Plants are somewhat more versatile than fish, with a few exceptions, and the choice of plants partly depends upon fish, though most fish are fine with any plants. Temperature may play a role, depending upon the fish.
To answer your question on rocks: calcareous rock (limestone, marble, dolomite, along with coral and shells) will add mineral to the water, increasing the GH and pH. If you go with hard water fish, this will not matter, but if you have softer water and decide on soft water fish, these need to be avoided. I use "river rock" in a couple of my tanks that are aquascaped to replicate a stream. These are safe, and come in various sizes to represent boulders, etc. I get mine from a local landscape supply, they are very cheap at 75 cents for a whole pail.