How much do levels and gear matter?

I've been playing for about 3 weeks now (and lurking on here for less than 2) and I'm close to getting my first group of level 50's (Tyro, Rinoa, Squall, Rydia, Kain). I have only Tyro's memory crystal and I'm running mostly max level 3+ or 4*'s for gear. I know abilities make or break nearly every boss fight and record synergy helps but how limited will I be by levels or gear? Like is it feasible to attempt some of the 75+ difficulty dungeons with what I have? I still struggle with Heroic dailies and have to S/L a lot to clear and + mobs pretty much one-shot me.

Sorry if this is a repeat question somewhere; I did a search for "level" and "difficulty" and didn't find an answer.

Edit: Thank you for the responses!