That grey looks older than 7 weeks to me also, are you sure the breeder said 7 weeks?
I'm not sure about converting the balcony to an area to house the grey, I think a cage would be a better choice but then I can't see what it looks like, I guess it could work but you would have to make sure it was escape proof.
The breeder should instruct you on how, when and all the whatifs about handfeeding but we also have several threads that explain it and weaning. You do have to be careful and know what you are doing, your bird's life depends on it.
Let us know what you decide to do about getting this grey but we will support you with guidance and advice if you do take it.