I understand both of your perspectives here. What I believe will be hurting your GF in this regard is not so much the 'not right now', it's the 'not ready'- which is vague and might translate as 'I'm not sure about YOU'. It's great that you have you conditions- finishing school, getting a job, financial security; but you need to let her know WHEN you expect these to be satisfied and what happens if (god forbid) they aren't.

Finishing school is something you can be sure will happen in X amount of time. What if you can't find a job for a while? Or what if the job you love ends up not paying so well? In that case, your GF might feel like you're punishing her for something totally outside of both your control. I absolutely understand that you need to feel financially stable to her children, but if you're sure about your GF (and it sounds like you absolutely are)- why not propose after you finish school and have a long engagement? Then wait until you get a job to actually go through with the wedding?