I'm assuming this is for PVE.
Necro is a kinda stigmatized class for pve, but whatever. I've done most all the hardest dungeon paths with a necro in the party. There are 3 I semi-regularly dungeon with. That said, I would say that a guardian generally is a great anchor for a party, if that makes sense. I would be more willing to join a pug with 3 guards then 3 necros, even though I know that's not exactly fair. I don't really play a loved class either by the puggles (ranger), but it's cool to play something that you don't see a lot of other people playing.
When you say exotic in the second part, I assume you mean ascended. I would probably get ascended weapons first since you only need 2-3ish of them until you want to complete your class's full weapon set. Comparatively, getting a full armor set or a legendary is much more time consuming and costly.
As for saving mats, eh, seems like a popular thing to do. That's up to you. I don't plan to obsess on hoarding stuff to get new shinies asap.
Yeah, I meant ascended, sorry about that. So if I have exotic weapons from dungeon tokens, is the upgrade to the ascended weapons that much of a bump or are those just wasted materials I should have saved for the legendary?