I've helped with the non vented to vented conversion on a friends 108 (280se 2.8), while doing so our research showed us that :-

1) Yes the spindles are identical on the sixes and V8s (above car proved it)
2) Having discussed it we are pretty sure it would, but the 63' spindles should also be the same taper ? (mercedes only redesign something if they have to).
3) The calipers are sized to the width of the discs as others have noted here, so if not buying re-cons (they're actually quite cheap) you want the calipers from a V8 car, whether that's a 107, 108, 109 or 112. We used 107 discs and calipers on a 108 with it's original spindles.

The only things that wouldn't fit were the backing plates, having said that we didn't need to get others we simply 'modified' them to give clearance using a couple of taps from a hammer.

I presume you're not doing the back but as far as we could tell the rear brakes are all pretty much identical in terms of calipers and solid discs with internal handbrake shoes.

Have fun

J