Here's what I would suggest. Don't cut the entire inside skin off. The reason is because it adds rigidity to the door and if you cut it off, even if you bolt it back on, it's going to change. Door skins are tricky business.
Instead, consider cutting "access panels" into the inside door skin. That way, there's enough of the door skin to maintain original rigidity.
As far as insulating your door, try this. Since you're having the inside of the doors custom-upholstered anyway, then apply something like Lizard Skin to the exposed interior door skin, and also apply either that or whatever sound-deadening product you want on the back side of the door upholstery also.
I will end up doing this myself. Because all the floorboard and firewall insulation in the world is not going to help much if the doors are still like hollow steel drums.
