Cage size needs to be as big as you can make it if you're looking into one of the big macaws. A bare minimum would be one of the large double cages in my opinion. That or an indoor aviary, still with tons of out of cage time. If you're interested in mini macaws then requirements will be different
A galah can live in a smaller macaw sized cage comfortably, but again, the bigger the better
How loud depends on the species of macaw all together. Obviously the larger macs are going to make a lot more noise. They can be heard from a long long way away. Even the smaller macaws can have one huge set of lungs on them
The galah is, well, a quite quiet cockatoo considering it is one. They remind me of large cockatiels in all honesty. Only a bit more bouncy! They can make some high pitched twitters and screeches but they don't have the RAWK of the white cockatoo
Birds love companionship and larger birds seem to feed off that even more, in a more emotional way. By all means, smaller birds feel it too, but these birds take a long long time to mature and stay babyish for a considerably amount of time. They need a lot of attention. You'll find the galah is probably going to be less needy than the macaw
Java stands with vet-wrap to make sure they're not too slippy

Buy a stand and then wrap it up, easy peasy
Although I don't really think the idea of 'starter' birds is a good thought, I do not think that macaws or cockatoos are for the inexperienced. However, the galah is probably the best to start with. Macaw-wise the Hahn's is an excellent choice. I do not want to sound patronizing and if you already have birds then I do apologize, but it has to be said. These birds are not for the faint hearted. A bite from a macaw is something to be taken very seriously. Luckily with the galahs, they have quite a small beak :lol: