Law / A D-visa required? Info regarding emigrating to Poland. [5]

Hello! I'll write this in English as my Polish isn't quite ideal yet. I'm a US citizen who's been long interested in moving to Europe. And after a few years of consideration in terms of job market demand and university options I've set my sights on Poland. I'd like to reside there while working and studying.

But one thing that isn't clear and all info I've found is rather contradictory: is whether or not I need to acquire a D visa before arriving in the country or if I can simply go with a passport and get everything handled within the 90 day visa free period. I've attempted reaching out to my districts consulates office multiple times to no avail and they're nearly an 8 hour drive from me so I'm trying to gather as much information as I can without having to go to such lengths. I work in the USA as a CNC programmer and am planning to study as a web developer/programmer as it's the line of work I'd like to go into. I have roughly 10,000USD set aside for this and have heard that one option is hiring a TRC lawyer upon my arrival which I'd have no issue with if it actually is an option. I'm just looking for some clarification on whether or not the D type visa is absolutely essential beforehand or if it's something I can forgo in favor of getting it handled once I've made my way there. Thank you for reading and your time!