I would say easiest to source and care for is generally Malawi, generally Mbuna. They are particularly unfussy about Ph/hardness once is over or around the 7.5 mark or so. Tangs I believe do actually really benefit from very high Ph (8.2) and hardness, and I think they are trickier to stock well in a 55 (ironically easier in say a 40 Long but once you have 55 you can't actually put that many more fish in).
Victorians are hard to find and the aggression is massive.
So I would reccomend you check out Malawis - what's available locally to you ?
NB all of the rift lake species are massively intolerant of poor nitrogen cycle management - nitrates must be kept very very low (shoot for 20 or less) and you need a properly cycled tank.
I have to disagree. Some species of Victorians have higher aggression, but some are not quite as bad. This generalization is similar to saying all mbuna are massively aggressive, which is not true.
I keep a few (like 10) tanks of Victorians, and while some do have aggression problems, most are within reasonable ranges...like keeping a species tank of yellow labs. There will be some chasing, and fighting at times, but nothing that can't be addressed with proper stocking levels.
Mbuna are much easier to locate, that is very true. For a first cichlid tank, I think you are on the right track with the species you are selecting.