There are members here with six and eight footers.. some over two feet deep, and they manage without a mask and snorkle
On very deep tanks you can use a grab for planting and moving things.
Some territorial fish... such as Oscars and other Cichlids will attack, but apart from the shock you get, and possibly a nip or too... they are pretty harmless.
Discus "and" corals come from different areas, so it is unlikely you would find both in the same spot. There is an "allowable list" of the fish allowed into NZ, and only certain persons with the correct autorisation are allowed to import fish.
Bigger tanks are certainly easier to maintain, as a small tank can become overcrowded quickly and the water parameters are harder to keep stable.
The thing is, that you will still need a heater, light, filter etc, and price for price there is very little difference in these items for a two foot, or a four foot. Buying second hand might be the way to go... but buy a "NEW" heater at least.
I see you are in the market for a tank...
Think about the size you want.... then forget it... then think about the size you "need" and your future plans... (community.. breeding..etc).
Major rule... Take your time, both in the purchase of the tank and the stages after that. In other words "do it right" and get things as they should be before adding your fish.
There's heaps of info on here about setting up and the "cycle".. so until you can afford that tank you should take this oppotunity to do so careful reading
HTH