It would need a focus limiter (maybe the old one already has this?).
If this is an all-around lens, I'd probably take 3.5-5.6 for the cost and portability factor. Not that 2.8-4.5 wouldn't be great, but the whole point of this would be a walk-around lens.
I shoot on a crop body so I'm not sure this would ever be a one-lens solution for me, but it might pair well with a UWA zoom. The amount I'd be willing to pay for it is probably a lot lower than what it would actually go for.
I'd guess if it was 2.8-5.6, it would be 2700, if it was 3.5-5.6, it might come down to 2000 or so. If I remember right even lenses like the Tamron 18-270 pushing $800, so imagine an L version of that, and you've got to imagine it will be ~3x as expensive.