I do a fair bit of aquarium photography and, for me, the key things are the lens and a flash that you can bounce off the ceiling. My aquarium is tropical freshwater, though, and I do not have the lighting that you almost certainly have on your reef setup so a flash may not be as essential for you.
I personally use a Nikon D3300, which is a cheap and excellent body. I'd personally avoid the kit lens for fish photos though as it will frustrate you.
A macro lens in the focal length of about 100m (I use a Nikon 105mm VR micro) will be exactly the sort of lens you will ideally get. You will really struggle with shorter focal length lenses unless all your fish are 6"plus. WIth something around a 105mm lens you can stay a few feet back from the tank, which usually leaves the fish acting rather more naturally than if you get up close (when they tend to either think you are going to feed them and all you will see is their faces or they are nervous of your close proximity and hide).
I need flash because of my lower light levels in the tank and this is a nightmare if you point the flash straight ahead because of the reflections in the glass. Bouncing it off the ceiling (with a transparent or open tank lid) gives nicer light and avoids these issues.
The Nikon 105mm micro VR lens is by far the best aquarium lens I have used and if you are going to go Nikon is what I would get. Sigma also do a very well thought of equivalent. Also superb for other macro stuff as well of course.
For me the D3300 body works great. If you want more frills (although the image quality is the same) then go for the more expensive D5500 or D7200, which will both give you fancier and better autofocus that will give you a slightly higher % of in-focus shots.
Also note that many fish really hate the autofocus assist light - turn it off unless you really need it.