The turnover rate for any aquarium should be in the 5x/hr range. If you are going to use undersized filtration, (2x75 rated filters) the bio may be adequate in the beginning, but be mindful that when the bug bites for more fish, bigger fish, whatever, the need for more bio will increase. I would suggest bigger filtration, or, in the least, good water movement, i.e, powerheads. Who wants to look at beautiful fish when there is a bunch of junk floating around in the water?
If you go for the aggressive African cichlid, sometimes the only way to curb aggression is to overcrowd, or species only. This will sometimes only lessen the aggression. These are beautiful fish, easy to take care of and reproduce like rabbits. They stir the substrate, dig and undermine whatever is sitting on the substrate and are messy eaters....more filtration.
There are tons of cool fish out there and a 120g is a great tank to start with. The bigger the tank, the easier in my opinion.
Jusy do your research and take it slow.
Welcome aboard!