No, it’s not a pay-to-win game. All gadgets are unlocked from the very beginning, and even weapons are open, so you can mix any weapon in any class. With rank progression, you do unlock a few additional weapons, but they’re in no way better than the default class weapons.
Your weapon of choice should be dictated by the situation and your personal preference. On any map, big or small, you can take advantage of the environment — whether you prefer low or high caliber weapons.
Most players who have been around since the first day of open beta have already mastered fast triggering, so I’d recommend avoiding open spaces, keeping to cover, and learning how to dodge effectively.
Personally, I’ve one shot more than a few people with an RPG - lol, so yeah keep charging and try to have some fun!