First time I post in here, so if I make any mistake please correct me so I can learn from it

So, I'm gonna play a campaign soon, which consists on 3 phases (300pt/600pt/900pt), where I have to buy, paint and play an army. This time I chose to use an evil army, since I do not tend to do that in tabletop games (always play as the good guys). I have to buy the miniatures for the campaign (I don't own any evil unit) and I don't have that much money to invest in a cool army with a lot of variety, so I thought going for an elite force would be the most clever move, since I would fill the points expending less than going for cheap infantry (+ will be harder to fail morale tests). These are the lists I thought for the 3 parts.

300 points:

Warband 1 5/12
The Dwimmerlaik with Horse;
5 Morgul Knights

Warband 2 0/12
Ringwraith with 1 extra Might, 2 extra Fate

600 points:

Warband 1 5/12
The Dwimmerlaik with Horse;
5 Morgul Knights

Warband 2 0/12
The Necromancer

Warband 3 0/12
The Shadow Lord

900 points:

Warband 1 5/12
The Dwimmerlaik with Horse;
4 Morgul Knights, 1 has banner and 1 has war horn

Warband 2 0/12
The Necromancer

Warband 3 12/12
The Shadow Lord
12 Mordor Uruk-hai with shield

Warband 4 9/12
Black Númenórean Marshall with armoured horse, shield and lance
9 Mordor Uruk-hai with shield

Since is not a tournament I don't want to go full competitive, so what are your thoughts about them? Would you change something? Just leave it this way?

just a question when you sale up the army from the 300 to 600 points there are you allowed to drop the generic wraith altogether and replace him, would you not have to keep him and just upgrade him maybe and then build off whats in your original starting list