Have you saved up for the mount? Don't waste coins on the bag imo. I say just keep farming creatures (Rares and sometimes elites) and chests for the pieces you need. If you desperately need the weapon from the vendor and you wont possibly get an upgrade in the near future for the slot then I say go for it. Coins are fairly easy to acquire, just time consuming. Seeing as there is LFR though, I personally would wait for a drop from there unless ya'know, you are incredibly unlucky.
I actually bought Mogu runes and ran some TDD LFR to grab multi-socket items for challenge mode/proving grounds gear. Sockets are pretty strong for those modes.
A socketed gem will not be scaled down. This is a huge bonus. Your stamina trinkets will be scaled down to 1270stamina. Your stamina gems will keep its 240 stamina. More sockets more survivability. 5 blue gems are nearly worth a full stamina trinket.
Don't buy the bag, you'll eventually pick up enough epics to fully gear yourself along with Burdens of Eternity to increase the ilvl.
If you're thinking about buying the weapon then yes you need it (in general a weapon upgrade will improve your dps over other gear), then collect more coins until you have 50k coins and get an ilvl 535 Trinket.
After that see what gear level you are at as to whether to save up for more Burdens of Eternity.