If you're using Aperture 3.3 or later, you can use "Auto Enhance" to do this, which will attempt to automatically set the white balance, exposure (a mild tweaks), put a curve on the photos, etc. In general, I find it provides a pretty good starting point.
You can do it as an import preset. OR, you can select all the photos you want to edit, then choose Photos->Add Effect->Quick Fixes->Auto Enhance.
If you don't like it you can go through Photos again to remove all adjustments.
Auto enhance is pretty neat; it has 4 or 5 things it does. You could define your own preset that did ONLY white balance and exposure if you wanted as an import preset.
Note you have to do it through the menu at top. If you do it from effects on the left it will only apply to a single photo.
Yes, it will. These are "smart" adjustments that are image-specific.
One note: I've found that auto white balance occasionally detects faces in photos where there aren't faces, and thus will white balance based on "skin tone" versus "neutral grey" and there will therefore be a pinkish color cast. Happens maybe 1 in 20 images. You'll have to reset the white balance on those images to neutral grey. Still, it means 95% of the time it's much better