Silent lens for a57 video?

Could someone recommend a silent a57 video lens in the normal range... it will be used with steadycam and Autofocus and I think that an external mic might make the camera too heavy. but if I use one I would like to see which mic is small and is quite cheap.

Im thinking about sigma 17-70 2.8-4 HSM or Sigma 18-50 2.8. Do you know if any 28-70 or 24-70 2..8 has hsm and is cheap (no more than 400 euros used)

The problem of the lens changing brightness when you zoom is real hindrance for wedding videos so if its not noticeable in the sigma 17-70 I might buy it since I need the longer reach.

sony 16-50 is too expensive.

Thank you.