Well... there's a pretty long-running debate over what gear is even necessary below ~40, since levels are pretty fast in parties. Without materia, lower-leveled gear (other than the weapon) often doesn't even provide enough of a bonus to make it "necessary" considering that it'll probably only last you ~3 parties. Don't get me wrong, it definitely helps, but you have to consider the cost (in time or money) and whether it's worth it. People won't normally kick someone out of an exp party for low gear, within reason. Level 40 with Level 8 gear would be pretty bad, but Level 15 with Lv.8 gear shouldn't bother anyone. If you're level 35 and wearing Lv.25 pants, but there's no gear between 25-35 that provides more of the stats you need, just stick with what you have. I wouldn't upgrade for defense alone. And if it's going to cost you more than you think it's worth for +1 or +2 MND, you can probably skip it, as well. Tanks are the exception; if you plan on getting hit, your gear definitely does matter.
There are a lot of drops from open-world NMs that have green names, and are definitely worth getting for the way up. For Conjurer, I would ask your LS to help you get:
- [item=5030304]Kple Kple[/item], a Lv.30 staff from Alux (Lv.42).
- [item=8032301]Loyalist's Bliaud[/item], a Lv.30 body from Downy Dunstan (Lv.42)
- [item=8010130]Divining Hat[/item], a Lv.30 hat from Bardi (Lv.42)
- [item=5030112]Bloodcry[/item], a Lv.35 staff from Spitfire (Lv.47)
I'd also say accessories are useful. You should try for
- Pearl Choker (Lv.38)
- Mind Rings (Lv.29,39,49)
Other than that I wouldn't worry yourself trying to fill in other slots; just buy what you can afford and keep your weapon up to date. There's a plethora of nice items from Toto-rak (Lv.25 dungeon) and Shposhae (Lv.15 dungeon) but in the time it took you to get them, you would probably already be Lv.35.
I'm sure I forgot something, and don't take everything I say as perfect. ^^;