OK, it's time for research, on a tank than size you'll need at least 2 filters and 2 heaters. If 1 stops working all your fish won't die while your fixing or replacing the other one. Also, If one does break buy 2 more. If one goes out the other probably isn't far behind.
Also, do you want just day lighting or do you want moon lights too. The LED moon lighting looks so cool on a tank that size.
The gravel depends on the fish. Not all fish eat gravel and there are pros and cons to both sand and gravel. If the fish your getting like a higher pH there are crushed shells that maintain an alkaline water, the shells look nice too, especially under the LED moon lights.
In one 55g tank I have natural looking small river rock, in the other 55g tank I have a mix of small blue and lavender gravel. I like both, even though they are very different looks.
But to help keep the pH up in my Cichlid tank I'm going to mix the shells with the small river rock.
There is one thing no Aquarist should be without, a water changer. Trust me you don't want to lug buckets of water back and forth, it's a huge PITA. I did it for years before I found out about water changers, what a revelation. I no longer dread cleaning the tanks.
There are basically 2 types of water changers and they work pretty much the same way. One is a Python and the other is an Aqueon, both hook onto your faucet and first it sucks out the dirty water and you flip a switch and it fills the tank with clean water. The water changers also allow you to keep the water temp stable during the water change. A change more than a degree or 2 will stress out your fish and give them all ick.
That's enough to make your brain hurt so I'll stop for now. After you decide on this stuff I'll tell you about more decisions you'll need to make.
Have a great weekend, LOL.