New game

1. The usual are the best for the main portions and some additional content (eg. Legend Mode).
The endgoal is to expand to have 6-8 of the best from each class and practically all gens because of the restrictions of numerous post-endgame and side battles (Princess Trails and Unit Trails). However, this can wait.

2. All the gold gens are featured in some of the above side and post-endgame content, IIRC. I'm not sure if they're good to have or must-haves, but all gold gens have their uses.
An all-gold team cannot function well on its own. Most of them are solo fighters, not team players, and this will backfire when the battles get much harder. Plus, they're the most expensive gens, pardoning one or two purple gens. When you don't get 400 medals (2 sets of 10 Daily Missions at 20 Medals each) + 240 medals (2 sets of 3 Daily Legend runs, assuming Decisive Battles are fought) at the minimum, investing more for a less effective team will hold you back.
Aura is the key and these are scattered amongst the other colours. Do NOT go for an all-gold team.

3. Read (1). Also, some of their skills are still quite good. Mocty and Li Hongzhang's actives are some examples of this. I use Li in my main team with equipment that generate rage per turn.

4. That is the easiest way. Otherwise, farm medals and upgrade your current core of 6-8 gens (or buy them if not bought and possible). You can also read our guides. If you performed exactly as said, but still fail, you need better firepower (upgrading your Age and gens, or invest in your gens).

Any proper recommended lineup. Just ignore Suleiman and Caesar (the better of the two, but needs a dedicated inf com team).

You need to tell us who you have and which type you're going for first. Otherwise, we can only tell you the generic recommended gens like Li Hongzhang, Nobu, Atilla + Saladin/Edward/Awakened Suriyothai, etc.

Atilla + Awakened Suriyothai + Edward/Saladin should be a good place to start.