A 3D printer would be a useful tool to have in your model shop, but probably not the only one you'll want. Knowing about modeling software will definitely help. Most downloadable models are in STL mesh format. That's not an especially easy one to modify, although some software makes it possible. You can, of course, always print things out and modify the physical models using other tools. Most printers come with slicing software (usually the kind that's free) but not with modeling software.