I agree on both counts. For what you're looking to do, size will be an issue as well, for most 3D printers can't make big things.
Inventor, while great, isn't a good option for organic surfaces (such as point clouds and faces). I'd recommend a more mesh-based modeler, such as Max or Maya. Some folks also use Rhino to good success for such things. We use Blender for that kind of stuff, which I love, but I'm crazy so you shouldn't listen to me. 😉
I'd go with a larger CNC table like what we've got, or better, so that you could do very large sculptures. There is some cheap CAM software we use that will 'slice' up models into layers that you can stack to make whole life-sized sculptures and such. I've seen some folks with the same machine we have make some 12' Egyptian statues and such for nightclubs, where they carve them out of foam, stack and glue the layers, sand it down nice, then either fiberglass or plaster over that and paint to finish. Kind of what you're talking about. Get on some CNC forums, like CNCzone or the talkshopbot.com one and ask around, you'll see what I'm talking about.
Hope this helps,
Jeffrey