Even if it is low D grade it will be better than top NG. I recommend doing this unless you still have a NG weapon with p.attak under 10. It may take a little longer, but you will make up for it when you can afford your D grade at about lvl 20.
But I guess it also depends on if you will be able to get D grade soon enough. I mean if you think you won't be able to get D grade until lvl 23, maybe a top NG before lvl 20 would be good.
cheapest D, is what, 500+ k? well, its not a problem if you team up with a party/friend and do some quests or just grinding together. saving that kind of money with a low-lvl wep isent very funny :(
There's an upgrade function for Novice characters -- if you can use it, you can upgrade without losing your previous investment.
The only no-grade weapons I see people buying and selling are the top NG. They sell slowly around 200k, I think.
Depending on your class, you can go deep into your 2nd class with NG gear. And, actually, unless you do something heroic (profit from events, level an alt, de-level to farm manor seeds, or Ebay) you will have to wait a while to afford full d-grade.
I'd say save up for D grade, because D maintains its value. every piece of D equipment can be crystallized, and the crystals sold for the items shop value. No grades, on the other hand, sell extremely slow.
--Edit: If you just stared the game, theres a bonus weapon for new characters available at the "adventurer guildsman". he will actually give you an ng weapon, and a D weapon once you reach lvl20. -so, use that, and theres no need to buy a D weapon - yet.