Potions serve more utilitarian purposes, and it's up to you to decide whether to make use of them. The most common is the Swiftness Potion, which can provide you a handy boost in speed while going after annoying quest objectives indoors. These are generally MASSIVELY overpriced, because people know how useful they are. I opted not to spend the gold since Warriors are already fairly mobile, but slower classes may benefit greatly from these.
For future reference, it is a great option to purchase mounts and pets as a long-term investment. Not only do their prices scale well even when a new expansion drops, but there is always a demand for them. The only drawback to this strategy is you would need a hefty amount of initial wow gold investment and also maybe have to go through the motions of spending for a guild bank.

I guess maybe I'm familiar enough with it to just slam it out. I don't recall any low droprate quests as bad as 5%. Although Blizzard has been screwing with stuff lately. Do you remember which quest it was? A lot of those can be skipped if they're too tedious. Like literally ALL the quests on that little island in the NE corner of the map with the ghost pirates underwater and the dragon whelps.
The Escalation patch has introduced several new scenarios with respective heroic modes that provide shiny new loot. It is still too early to predict which of these will demand the highest prices at the auction house or AH, since it can take weeks for players to work through the content and determine the value of the new drops compared to the existing items. Many expect the rarest and most prized drops to start commanding handsome prices starting June, so just keep your eyes peeled for popular WTB requests in general chat and high-selling items at the AH.

It's not an enchantment, but you should buy or craft an iLVL 100+ belt, gloves, or boots with a gem socket for use with the new 5% bonus experience gem: Insightful Rubellite. I used a vendor-bought belt from the legacy valor vendor in Orgrimmar, since it was convenient and met the minimum ilvl bonus. Keep these in your bags until you can equip them, which should be just in time for treasure collections in WoD content. I didn't ever replace the belt until I dinged 110, since I didn't feel the stats from later upgraded belt drops made enough of a difference to outweigh the 5% bonus. This is explained below, in the WoD treasure section.
Fourth: This is very important; do not vendor the gear you obtain while leveling. There is a new mechanic introduced in BFA called the Scrapper which allows you scrap your gear for trade goods which will be very important for rep and AP grinding (will talk about it more later). Usually after I clear a zone, I will head to the scrap bot and scrap all my greens/blues to clear my bag for the next zone.
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.