Note: My goal here was to create a comprehensive list of everything you'd need in order to learn to play any class (from Level 1 to Naxxramas)—meaning I am only including class-specific resources here (so no profession guides, general leveling guides, etc). Teeb's Resource list is much broader in scope, and I highly recommend you take advantage of it (as it contains a plethora of non-class specific resources as well). If you're not aware, Teeb is also responsible for the Race & Class Comparison Tool, Dungeon Tier Sets Gallery, and a huge variety of other insanely useful resources.
In terms of what the World of Warcraft Classic Best Classes are, Warrior and Priest stand out almost instantly. This is partly due to the fact that Protection Warrior is the only viable Tank Class in WoW Vanilla Patch 1.12. Both the Proc Warrior and Priest are required for Raids and when combined in a PVP setting can be a fierce combo on the battlegrounds.
A demo of the game was available at BlizzCon 2018, and was downloadable on home computers for anyone who purchased a BlizzCon ticket or virtual ticket. The servers became available when Opening Ceremony started at BlizzCon 2018 and was set to end on November 8, but was extended until November 12. Players started at level 15, and the only available zones were Westfall and the Barrens. It was based on patch 1.13.0, essentially patch 1.12.0 ported to a modern infrastructure. The first day of the demo, there was a playtime limit of a cumulative 60 minutes with a cooldown of 90 minutes, applied through the BlizzCon Exhaustion debuff. The debuff was removed on the second day of BlizzCon 2018.
So even though you can play a WoW Classic demo today, we’re not done quite yet. We have lots of capital city features to look at, such as banks and auction houses. We need to test our dungeons and raids to make sure the bosses’ abilities all still work correctly. We need to examine all of our PvP systems. But we’re committed to taking a close look at all of these and more as continue bring the classic game back to life.