Thank you everyone for your advice! I'm sorry if I came across as someone who hadn't really put much thought into this above in the first comment; I have actually been thinking in detail about this alot lately; I just forgot to put it into the above post
I found a website recently about a Lab in Spain that will do a test for you for all the common parrot diseases relatively cheaply, and I was thinking maybe I would ask people beforehand to do this test.
Caging: I am going to designate a room in my house as the 'bird room' with only birds and bird stuff in it so I have the space to look after the parrots. At the moment I have a spare small budgie cage, a large-ish flight cage that could easily hold an Indian ring neck or a parrot of similar size and I am going to get my dad to build a large flight cage that can hold a macaw. If the cage is transportable then they can bring their own.
Food: I would ask anyone to provide their own food to last the duration of the birds stay so that the parrots don't get upset stomachs from the change of food and feel more comfortable. I'll also ask the owner to provide their own food bowl so that the bird again will feel more comfortable and also to prevent the spread of disease if it did turn out the bird had a disease that wasn't detected.
Toys: I would provide toys (I make them myself)
Health: I'm not sure yet about what I would do if the bird suddenly showed signs of being ill. Obviously I would immediately contact the owner and then take the bird to the vets but if it died I'm not sure.....
Weighing: I have scales that I weigh my birds with at the moment. I will weigh any residents everyday so that I know of any weight loss that may indicate illness
Fresh food: I cook everyday for my birds anyway. Broccoli, beans, rice and pulses will be provided every day as well as many other things.
I hope I've answered your questions. Is there anything wrong with what I've put above? Anything I could do in a better way? When I say tips on handling larger parrots I mean like are they the same as handling smaller parrots? Just bigger?