Mechs in general won't be particularly good for starter AA, I think. If the mech is light enough to be available early on then it doesn't have enough tonnage to mount the weighty autocannon batteries. AA mechs in general are fine, but I think tanks will still generally have the edge in that field.
About the AA turrets - How about automated turrets with 1-2 LBX-AC2(s) and a high declination limit? Unable to target low flyers at long range, so only used for minimizing ballooning. Ballooning would still be possible since LBX2s do so little damage, but it would discourage them from making a habit out of it.
On a side note, look at the main bases on Kagoshima and Death Valley. They appear to have "Eagle Eye" AA turrets already.