This is a rather short advice for people, casual or not.
If you plan to keep your gear in order a week from now, start buying gems, and especially enchanting materials, now. Preferably you started yesterweek, but there's still time.
The same advice goes for whatever materials crafted gear which you plan to aqquire needs for completion.
With the new expansion a sudden influx of upgrades will swamp the servers, and whatever is currently stocked up on the AH will be woefully insufficient to meet the needs during the first weeks.
One item in particular will be very hard to come by, and that is the enchanting shard from disenchanting blue quality gear. Players tend to run the troll instances rather than the old Cata heroics, and the troll ones only drop green and epic quality gear. You may want to spread your VP grinding between old heroics and troll ones only to get a chance to aqquire the enchanting materials.
Oh, and do get the materials needed for the new tanking leg enchant. Plate tanks won't even have to wait for an upgrade to feel the need to get that enchant.
Head over to MMO Champion to get a heads up on which upgrades are available to you without you needing to raid. For the same reason, if you aren't already decked out in all the VP gear that you need, remember that those will be available for Justice Points in a week. Doing this will give you a rather good idea of what gems and enchants you're likely to want within the short future.