You've got GC gear pretty much correct.

Tomes & dungeons are mostly reversed, though. Tomestones of Soldiery give i100 gear, which can be upgraded to i110 (via Hunts, Syrcus Tower, or 2nd Coil). Tomestones of Poetics give i120 gear which can be upgraded to i130 (via Final Coil). The highest gear you'll get from a dungeon is i90.

There's also:
Syrcus Tower ("easy mode" 24-man raid): i100 armor (can't be upgraded)
Second Coil: i110 gear
Ramuh Extreme: i110 jewelry
Shiva Extreme: i110 weapons, jewelry
Final Coil: i130-135 gear

Coil & ST are raids, the Extremes are arena boss fights, so I'm not sure if you're putting those together with dungeons. But the lines get blurred a bit among all those things. Your likely progression will be to use a little crafted/GC gear to get into the earliest 50 dungeons, then mix and match your way through dungeon drops & tomestone gear, work on your relic and possibly get a few weapon drops from primals and/or coils (if you do coils) and/or tomestones, working your way through more difficult content.

I wouldn't worry too much about regretting things. You might have to spend a little more time on something than you would otherwise if you buy something "early," but it's not likely to be a massive loss. Going from fresh 50 to raid-ready happens fairly quickly, and nothing is irreversible.