Plague Doctor has haste now, I would say max attribute is fastest class in the game.
New level cap is a few weeks away, so I would focus strictly on leveling right now, by the time you hit 390, the new cap will likely be in or extremely close. I would start for now by working on 380 primus (orange gear).
All gear now that drops out in the world can be identified and has random stats on it, you will find a ton of this while leveling. Everything you can’t use, you can turn into powder at a blacksmith NPC. The powder can be used to reroll the values (not the colors) of the sockets on the random stat gear–but I would not reroll anything until 380. Only ID stuff you can equip–you can dismantle even if you have not ID’d.
Scouts are great, you generally speaking have 2 paths: Dagger or pistol. The core damage class for dagger is assassin, and for pistol is bullet marker. Most other classes are filler, try to pick classes that will enhance either of those. Most people pick linker (see number 2) because it works for everything, and is also exceptional. Then most pick a random filler. For dagger, many do rogue or shinobi. For pistol, many do SR . Corsair/Outlaw/Enchanter/Thaum work well with either pistol or dagger.
There are also 2 new scout classes coming, sheriff (pistol) and arditi (dagger). Both look like a lot of fun. Those will be added when the level cap is increased in a few weeks.
Important change to the class system is that you can fairly freely change your classes. You can’t change your base class, but you obtain points from doing dungeons/raids/events/other activities, and every 1000 points you obtain lets you swap one of your classes out for another one. So if you picked corsair but don’t really like it? Swap it to arditi when it comes out.