Apprentice 100% required, otherwise you lose a phase. Plus they can cast spells (XP) and activate war band members.
You are after treasure, so a couple of thieves (Move 7) so they can lug it off the table.
You don't want your precious wizard or apprentice killed, so a couple of thugs to act as meat shields, and or lug treasure off the table.
A couple of archers are good for a starting war band.
So that's a core body of 8 already.
Left over expenditure is up to you, what models you have and how you want to play the war band.
After your first game, if you can afford it, buy a kennel for your base, and get a war hound (an 11th member), they're totally expendable but can harass and delay the enemy.
As others have said, if you have a Necromancer, raise zombie is almost mandatory. They can lug treasure off the table (slowly).
Bone Dart, Elemental Bolt or similar are all good fun spells for ranged attacks.
Telekinesis, Fools Gold, Reveal Secret, Brew Potion are useful spells.
HTH