Agree with people above, but I'd like to add something:
Spend time with as many different species as you can before deciding which bird you want, more than one of each if possible

It might surprise you which species you fall in love with!
You might spend time with conures and fall head-over-heels in love with them!
I love macaws, but I wouldn't want to own one, even if I had the time! My lifestyle/personality is more suited to black cockatoos and other 'lazy' birds, who don't mind just spending some time playing by themselves or cuddling with you.
Of course that's just me - you might meet some macaws and love them!
Of course there are differences between macaws as well, the feisty mini-macaws, the playful B&Gs, the gentle GWs, the zany scarlets - they're about as different as different gets.
Looking at life-spans is all well and good, but only if the species and bird itself are compatible with you! No use in living 60 miserable years with a bird you don't like
Also getting an older bird can leave you with a lot of baggage. There's nothing "wrong" with birds that have had a previous owner, but they can sometimes be left with bad habits and trust issues. Why is the bird being adopted out? Has it picked up behavioural problems? Health problems? Problems can be worked through, but it's difficult, and can be tough on both you and the bird.
In regards to how much time you'll need to allocate for multiple birds - it REALLY depends on both you and the birds!
Whether the birds fight when they're out together (necessitating individual playtime), how much time it will take you to get food ready, make toys, clean cages, etc.