So we keep the helpful information going in this, please concentrate on your next move,
@QQ7007.
You will buy the £100 PC from him, so what will you do with yours? Is the case you have better? You can mix and match things if you're lucky enough, but that will require you to tinker a bit. Would you be comfortable with that?
Do you think your cousin can help you modify this PC he is selling to you and assemble a new one?
@Wolfshadw put a good list of minimal components that you can just use for under £300, so you will still be under your budget for a full PC. I'd say that a great opportunity for you to seize.
Cheers!