[Imp]

I remember this scenario now you were unlucky the Gepards are very effective, low ammo load though. It wouldn't be worth including the air if you had more AA.
I buy my AAA exactly as Andy mentioned normally 2x SPAAA & 1x SPSAM section for a battalion in a typical side battle/campaign.
Defending, delaying I might buy extra AAgun SAM or MANPAD sections.
My view my SAMs are 1st defence vs planes with stand off weapons
AAA guns are good all rounders main defence vs helos.
MANPADS efficiency varies depending on time frame & conditions, main use probably bothering helos to stop them getting a good shot.

On the occasions you find yourself facing a dozen planes & often supporting attack helos dealing with the air hampers operations drastically & requires a lot of thought. That's when I do things like restricting MBT AA.