Darock, I'll try to answer all your questions the best I can...
Any bird has the potential to become quite nasty... understand that in everything you do with your bird....every time that you interact with him, you are teaching him something and therefore developing his personality.
How he is treated and taught will have a big factor in how friendly he is.
that being said, what you actually buy depends on the bird and your experience and confidence level. For some people it is better to go with an older well adjusted friendly bird.... this is because he has already been taught several good things as a youngster and was handled by someone who already knew how to teach him well. Expect to pay a lot of money for a parrot like this. ...it is like buying a well trained dog.
Or buy a baby. I do not reccomend you handfeeding yourself....leave that to the experienced breeder....buy it when it is weaned and can eat on its own. A baby lets you bond to it much more strongly and will learn from you all its life. But you do need to be pretty grounded in what you teach it.
Here is an example of what could go wrong:
You are teaching your young B&G to step up onto your hand..he gets frightened and bites you....hard. You jump back and yell ow! and decide to leave him alone for a while.... the bird saw the reaction and realizes that biting you got you to leave him alone. So later, he bites you again to make you stop and now you scold him....
2 things just went wrong here.
You first sent hm the wrong message that he can bite you and "teach" you to go away .... then the second time you confuse him by trying to teach him it is wrong.
This kind of unsteady interaction and fear of being bitten can likely lead to a mean and confused bird.
So in raising your own young bird....decide early on what you want your bird to learn and set out to
fairly and clearly teach it to him...always staying on your own set of rules.
If a bird learns that he can ever win a training session by biting you...they do become nasty biters...all birds....so it is important to not let them think you are afraid of their beak.
When Holly was learning step up, she always bit me. She was 20 years old and not handled much at all when I got her. She had a lot of fear and had to learn everything. I first got her used to touch before getting her to step up....eventually I used a dowel to step her up so that I was not getting ripped open.... then used the flat of one hand to "press her beak" while my other arm stepped her up. she steps up easily this way and soon I will not have to press the beak with one hand..... it works because it takes the beak out of the equation. Their beaks are their only defense so if they know you do not fear it, they have nothing else to use against you.
So the point is to never make them think they have to.

Always be kind to him and always teach him right from wrong, and handle him every day. You will find a baby grows into a very loving pet when taught well and fairly.
On the underside of buying an older one .... you never know how it was treated or neglected and you may end up with a mistrusting bird who bites out of fear .... these birds really belong with someone experienced. .. This was Holly's case, and I can share her story if you are ever interested.
You will find a large range of prices on baby B&G's. There are some very good breeders here on the forum too. Judge on the health of the birds, the knowledge of the breeder and their willingness to help .. and of course, how good their health guarantee is. A good breeder is always worth paying a little extra money for. FYI
If a bird attempts to bite you .... you may get a lot of different opinions here, because peopel train their birds differently....
But here is mine:
The first reaction to a bite is to pull away ... it hurts! But to a bird, this is giving him power over you. Instead, the best thing to do is what the bird does not expect and push just slightly forward towards the bird and say "NO" sharply each time....yes, it hurts, yes, you may be bruised and bleeding .... but it will happen more often if you do not teach your bird not to bite.
Then do again what you were doing before....as long as it was nice. Some people poke their birds and annoy it...the birds bites and they yell at it and keep poking it....

I think we all see the problem there.
Holly got poked a lot by her old owners. .. this causes more biting and mistrust.
Just teach ... reward ... play ... and hold often. Reprimand only when necessary and get right back to the good stuff. Birds are smart and remember everything. Always leave their side on a good note.... it makes them want you to come back later.
