I have kept different mbuna combos, peacocks, small haps at different times in 55's. I now only keep mbuna in 6' tanks and have changed both my 55's into planted community tanks. I say all that to say that if you want a "Happy Healthy Mbuna Tank", and this is your first - I would buy one large group of a fairly peaceful fish, like 12 yellow labs and keep them in your tank with a syno or three (depending on species).
See how it goes, see how you like it, then decide if you want to really dive into keeping mbuna.
I kept a successful all male mbuna tank in a 55g, but it took quite a while, and several deaths/removals to find the right mix.
If I were to convert one of my 55g back into a mbuna tank - I would do a group of afra (2m/10f) with 3 syno multis.
I have found the most interesting/pleasing/healthy-happy behavior in mbuna when they are in large species groups in large tanks.
Have fun, and don't spend too much money on your first stocking as you will certainly change it.