Can I get NIP before I am employed (just to have it ready)
I don't recommend even trying. My brother (who's a born and bread Pole, a student) wanted to spare himself the hassle and get NIP before searching for a summer job. He applied for NIP without stating the reason why he wanted it (that is, he didn't include any employer's details on the application). Insted of getting the NIP number, he was called on to explain himself; most probably they suspected he'd been "working black". It took four months of legal correspondence between the tax office and him before they finally understood that he did not in any way break the law nor intended to do so.
He still didn't get the NIP number, of course.