Depends on your relationship. If these are ornamental and meant to be a bonded pair, you likely won't be part of it. Lot of space to fly. If you want them as shoulder pets, it doesn't have to be that big but you have to make that commitment.
They can be loud, but are surprisingly one of the best talkers in the avian world. Budgies can really belt it out when upset, but when satiated, they make the most wonderful sounds and can have a conversation with you. Again, what do you want from these birds?
Yes, but they shouldn't throw seeds everywhere because seeds aren't that good for them. They should have a variety of foods, Harrison's pellets are the best. Also lots of greens, specifically liver calming ones. Aussie birds in captivity often die from liver issues. Feed dandelion greens, never spinach. Eucalyptus if you can get your hands on it, or even a few tea bits in the water. Tanins bind to excess iron in the liver.
Yes, trim the tips of the primaries and secondaries. They're nomadic and not homing. If they fly out the door, you likely won't see them again. These birds are designed for flight - they're not a chubby African or S. American parrot - parakeets are streamlined. Some of the bigger ones don't need as much trimming, but I do think most captive birds need trims to keep them safe and to keep their manners in check.
Yes, Liver disease and vitamin A deficiencies.
No
Breeder.