Hi! Here's my thought. I worked in an exotic bird shop while in high school, so I learned a few things via personal experience and (eek!) trial and error. After high school, I purchased a Timneh Grey and I got her as a very young bird, just few weeks after weaning. This helped me mold her behavior. Greys are pretty fiesty, and if you get one that's older, it will probably have some bad habits already developing.
As far as buying advice, I can't really offer any; my Grey was purchased from my aunt who used to breed them.
I really prefer the Timneh Greys, they seem to be a little more even-keeled, and they are a bit smaller. Good luck on becoming a Grey parent!