So I would suggest a 55+. What I would do is start looking for tanks on craigs list buy what you can get a great deal on that is 55 or larger.
I take it you are thinking Africans? What city do you live in? what kind of access to specialty shops do you have? How much are you thinking you want to sink into this project?
Hap + Peacock tanks are really nice.
You could also go specific to a lake like tanginika (sp?) but that will require access to either a great breeder + fish club OR some really awesome fish shops
If you go mumba you have pretty wide options, they are more common and much easier to find.
Tropheous groups are also awesome but they should be kept species specific.