Leatherworking has three specializations: Elemental, Tribal and Dragonscale. I’d pick up Tribal one. It allows to craft Devilsaur set items. Hunters, Rogues, Warriors, Feral Druids, even Enhancement Shamans and Retribution Paladins are among your potential clients. On any realm there is Un’Goro Mafia on Horde and Alliance, they control whole Un’Goro Crater zone and farm Devilsaurs 24/7. They are making thousands of gold. With Dragonscale Spec you are able to craft Black Dragon Mail set. Good demand as well among DPS classes Warriors, Hunters, Shamans, Paladins, for those who wants Bis Pre-Raid items fast and easy.
As you level, you will buy new ranks of spells. If you splurge out on an item upgrade, you will not have enough gold readily available to upgrade to the latest ranks of spells: placing you behind. Moreover, you can buy gear upgrades from the Auction House (AH) or vendors. Furthermore, you may have large investments that need to be paid for professors. You will need to purchase reagents and items such as leather, bolts of cloth, alchemy supplies – unless you have a charitable friend or another high-level alt.
And for talent builds. I prefer to make my own talent builds. But I've been playing RPGs since the 8bit days so half the fun is building my own characters up. I already made and bookmarked 1 deep fire mage build, 2 elemental mage builds, and 2 deep frost mage builds. And I might remake them tomorrow if I read up on any good talent choices I left out my 1st 5 times lol. And I have multiple builds for the other classes I play too.
Always sell all items on Auction House. If you are not sure about its value, then check the market. There are some risks that item won’t be sold and you will lose money just putting it on Auction House. Good idea to make bank low level characters and keep everything there until the weekend. All sales increasing on Friday, Saturday and Sunday. Besides, you can advertise in a trade channel to inform people that an item you are selling has been put on Auction House.