I was asked this question and was not sure 100% about how to answer.
As I understand, in XようにY X is a state/event that is beyond the control of the subject Y (a certain consequence X will arise as the result of an action Y), while in XためにY, X is an action that can be controlled by the subject of Y.
In some cases then, ~ように and ~ために are interchangeable, such as in the following excample:
In this case we can use either of the two since the subject of the main clause (the teacher) is different from that of the subordinate (the students). Therefore, he/she cannot necessarily control the action in the subordinate.
However, given rules above in the following example only ~ために can be used:
(With the X mark to indicate that it would be wrong).
The question I was not sure how to answer was: "What if instead of 買う with conjugate using 買える and say:
Would this be correct?"
(Notice that there is no question here about 買えるために).
I think that this makes sense, and it sounds correct to me, but somehow reading the grammar rules confused me a little bit (Maybe 車が would be more correct, as a comment made me think). I would appreciate a confirmation. Thanks.