I hate to say it, but if you "can't spend much" then most likely a large parrot is not for you. Also, please don't take this the wrong way, but I think we can tell by your questions, even just the size question, that you have not done NEARLY enough research yet to take on an Amazon. Yes, you have to have your first big parrot in order to get experience with them, but you need to RESEARCH first. Also, about not having a lot to spend... $400 for the bird... how much for the vet exam? Say another $400 for a few routine tests? And the cage...NONE of the cages sold near me for that price should house and Amazon, in my area you would be looking at at LEAST $700 for a decent cage. Amazons need a lot of toys, it is pretty easy to drop $100 on suitable toys at the get-go, plus a good healthy diet of maybe $40/mo if you are really frugal... and in the first month alone a responsible person is looking at over $1600, so "can't spend much" is not a suitable situation for a parrot like this.
Also, your question about training is the most concerning to me. Being an "animal whisperer" is one of the most DANGEROUS ideas someone can get about themselves. Yeah, my husband has "a way with animals" but he gets bitten sometimes because he has not STUDIED animal behavior, and does not UNDERSTAND HOW THEY THINK. Yeah, for some weird reason a lot of them seem to like him for no reason, and that encourages him that he has it all figured out, and then he gets bitten, and sets me back in my work with a bird because he has no IDEA what he is doing.
You asked if a bird that age can be train. First lets get the whole talking thing out of the way - some birds never talk at all and if that is not ok with you, don't get a bird. Also, if she talks now unclearly, she will likely continue to talk in a garbled fashion.
On to more important things - you asking about her age is a VERY CLEAR SIGN that you do not know how to train a bird, and have not researched it. BIRDS NEED TRAINING! They do NOT think like dogs, cats, horses, or people they think in a way that is very unique and takes effort to understand. If you try to make them think like you you will traumatize them, and they will become a "problem bird" due to no fault of their own! Again, I come back to RESEARCH. You OWE it to the bird to know what you are getting into and have a plan.
-What if the bird screams all day? What if she screams all night?
-What if she eats your computer, or chews up your window sill?
-What if she needs $3000 of vet care after eating those things?
-What if she pulls out all her feathers? Some birds even CHEW HOLES in themselves when their needs are not met.
-What if she bites, and I mean BITES everyone who comes near her?
She is, after all, a wild animal, even if she has been hand fed. Parrots are not domesticated.
Also - this man is claiming that this 12 year old bird has never bitten anyone - He is LYING. Any 'Zon owners out there want to disagree? I will happily take it back if anyone out there has an amazon (dare I say, any parrot) that is sexually mature and has never bitten.
Also, YNAs are not cheap birds - if he is selling this one for cheap, you can bet this week's lunch money that there is a problem with the bird. There are some RARE occasions where a non-problem bird is rehomed for a very low fee, but nine times out of ten there is a REASON for the low price - you know what they say; if it sounds too good to be true, it probably is! Also - why didnt the GF take it if it was her bird? 12 years of no biting or other issues does not *typically* lead one to abandon their companion to the whims of an ex-boyfriend.
I do not want to sound harsh, but we have all seen this situation way too many times - someone thinks they want a large parrot, so they go and get one without doing the RESEARCH (how many times will I use this word? AS MANY AS IT TAKES!!!) before getting one, and they end up with a miserable bird who makes them miserable, and then the poor bird is shipped off to a new family to start the horrible process all over again. Don't get this bird. Do a few hundred more hours of research, the good the bad and the ugly, and then decide what species to get, save up for it, and don't go bargain hunting in the parrot section. We all wish you and the bird the best, but I think so far we also all agree this sounds like buying a bird on a whim that you know nothing about. It is a recipe for disaster.