First you say it will take some time to hit 40. Then you say you're not patient enough to wait the 2-3 weeks before Gracia comes out. Seems kinda mixed up to me. If it's gonna take you time to hit 40, then you have time to wait.
You need to adjust your thinking, IMO. D-grade armor is not an investment. It hardly even matters what gear you wear at that level. You could wear a pink tutu and still do OK. D-grade armor offers very little benefit at all, other than slightly higher p.def. Nothing that really helps you level faster or better.
Where armor really becomes important is in the set bonuses. At D-grade, there really aren't really any good set bonuses for melee DDs. I suppose, if you can't get Clan Oath Armor, Brigandine is the best choice, but even that isn't really all that amazing. It gives you more p.def and more hp, but as a glad you're going to be looking for sets that give you better kill speed. None of the d-grade armors do that.
At C-grade, go Plated Leather. It's Light Armor, but it's hands down the best armor for glads throughout C-grade (and a lot of glads even prefer to stay in Plated Leather all the way till A-grade, skipping B-grade completely.) If you decide to go B-grade, Blue Wolf Heavy is the preferred armor for glads. In A-grade, you'll switch to Tallum Heavy.
It's 20 lvls to wear the armour so I wanna invest in something good, it'll take some time to get to 40.
I just want to stress to you that you've got it all wrong. The BEST investment you could possibly make is to SAVE your money, be cheap at D-grade and start saving up for the more expensive C, B and especially A-grade gear. I have got to get this point across to you. Wear Wooden Armor until level 30 if you have to! Don't waste your money on D-grade armor that does next to nothing for you. This game is as much of an adena grind as an xp grind, actually some people argue that it's more of an adena grind than xp.
Leveling 20-40 doesn't take that much time. With newbie buffs and increased xp, it goes pretty quickly. It's not at all an investment compared to C, B and A grades. It's just like a pit stop at some dingy gas station. And when Gracia comes, it will be even faster.
Gracia is just a few weeks away (I'd guess 2-3 weeks). In that upgrade, you can buy D-grade gear from an NPC for a fraction of the cost it is now (like 6k vs. 100k). The best thing you can do is to save your money up for C-grade and beyond. If you can do that by getting in an academy and getting clan oath armor, great! If you do it by wearing NG till Gracia comes out, that works too! Just don't waste your money trying to buy 'good' D-grade gear - because there is none!