Since you are starting with nothing and building according to your own personal desires your options are huge. I would approach it from one of two angles... and to decide which angle would suit you best ask yourself this base question... 'Which is more important, having your favorite cichlid? or having your tank’s appearance a certain way?
1) If you are dying for the tank to look a certain way... then design it all out (without purchasing anything). Do the research, make sure that everything you want is available and within your budget. Then find out what kind of fish will thrive in the desired environment you created.
2) If you would rather chose the fish, then create the environment to suit, it's time to pick your base fish. I would suggest doing a Google (or comparable search) for these two words with no punctuation: cichlid profile. This will keep you lost online for hours looking at pictures of fish and learning about their nature and requirements. Search far and wide and learn as much as you can. When your done pick your favorite cichlid (No, only pick one!) and find out what kind of needs it has.
Chances are whatever you pick could live happily with a sand substrate. Off the top of my head I can't think of a fish that 'needs' gravel although some seem to do better in it.
Be careful asking a forum as large as this one questions like "what should I put in my tank?" Each one of us will post an elaborate answer to what we would do with the tank if it was ours... but... guess what... it isn't. It's yours. All yours. So do the research. Something will reach out and grab you. There are several fish I would suggest you not pick as some are more delicate than others, but I'm not going to tell you what they are because you will fall in love with them and want them That’s just the way it is. We all want what we can't have (and some people go to jail for taking it anyway).
Also while your researching read up on fish care. Especially things like cycling your new tank and water changes. Your at a great place to be in fish keeping ENJOY IT!!!!
