hiya,
i'm so glad you are considering a budgie, birds often get overlooked for furry pets as a lot of people don't realise just what characters they have.
i'm not sure at the moment quite what information you are after, so i will just give you a brief overview for now, and then if you have any questions feel free to ask away.
as lindsay said my husband and i have quite a flock of birds, we have a large indoor aviary, not to mention a number of other cages spotted about the house!i have been keeping birds for 27 years now, so like to think i have quite a bit of experience, but like everyone i'm always out to learn more.
amongst my flock i have 13 budgies, and budgies is where my passion started when i was 7 years old.since then i have expanded to diamond doves, tiels, and canaries.
be warned once you have a pet bird they can very easily win you over - my husband was scared of birds when we met several years ago - at the time i had two, these days he adores them as much as me, to the extent that it was him who decided to get the aviary.
i have kept single budgies before now, and as long as they get plenty of human interaction they have always been chirpy. these days we keep our budgies at minimum in pairs, but that is purely because with so many one to one attention becomes harder so it is fairer to let them then have their own company as well as the additional company of us (we have a single male tiel however who is the baby of the house and regularly sits with us for "family evenings" on the settee having head rubs).
the good thing with budgies is if you got one, but then thought maybe they need a friend, they are both easy to come across, and easy to bond (we personally have found males easier than females).
cost wise the biggest expense is the cage, and as you say in your post, the bigger the better, although the birds still benefit from out of cage time.once you have the cage budgies are very cheep pets. we have occasionally had to make vets visits, but out of the birds we have/have had there hasn't been many had to go, and a lot of things a good avian vet will show you how to do yourself (we now clip claws, and in the rare event of broken blood feathers - something we hardly ever come across with budgies more with the tiels and doves - remove these ourselves).
budgies are very easy birds to look after, a nice starter bird. they live on average 7 years (although we have had one live to 12 years, and i believe the eldest on record was in its 20s!).
each budgie has its own character, females are a bit more destructive, males are noisier (though nothing in comparison to some of the larger birds - the budgies we find have a nice happy chirrup)
as for what sex/age a lot depends on the reason for buying a budgie....ie are you after a bird you can handle, are you wanting to try teach the bird to talk etc? or are you wanting the bird more as a companion to hear whistling away?
let me know what your looking for, and i can then advise more, as sexing a young budgie can be fun (the colour of the cere is used to sex elder budgies, but it varies depending on whether you are looking at a standard budgie or a pied/yellow face type I or II, lutino or albino, younger budgies are even more fun but there are some tell tale signs).
i will leave this post here for now but please ask away any questions at all and i will see what i can answer.
beckie