What gear to upgrade

If you mainly PK or care bout your gear being intended towards it, get the R9T3 wep. Its simply massively better than anything else obtainable in game currently. The other problem you got is extremely low m.def. it needs some upgrading. Likely best bet is going for R9r ring and +10ing that bugger for some extra defenses. NW neck is nice but extremely costly, you can get and refine R9r ring decently, which is more significant upgrade, for the cost of just getting NW neck w/o refines.

Cards might be the best upgrade path if you land a set, the stats gained would be pretty massive but problem is how random and expensive landing sets are. Leveling your cards would be beneficial too but I`d rather spend the coin on other things as you get fair bit of free C packs from dailies/heavenfall tower assuming you do those things.

But in short you dont need just one thing but several things and I would likely look into merching with the coin I had if I were you. But as far as priorities go, I would get R9T3 wep first, then r9r ring. After those i think best bet would be to level cards, least the A cards that are spirit and m.def. Continue doing FSP daily for coins is possible, keep rolling A packs, sooner of later you land some kind of set, which will be a nice boost. I would go for armor refines only after leveling cards unless you get some free bound orbs from some promos for example. Even if armor refines arent bad option.

The things I`d forget for now are NW neck, stars destiny, helm/cape upgrade and tome upgrade.