You could import PNGs of the items, and have the transparency in the image. If you want the item to always face the camera you can make it be a Component, and when you do that you can check a box to make it always face the camera.
If you have a model you select the paint bucket, then hold down Command and use the eyedropper to pick up a color from the model. In the Colors palette that appeared use the drop down menu to choose Colors in Model, then you’ll notice that the color you picked on gets highlighted.
Right-click on the highlighted swatch square and choose Edit. You can now move the Opacity slider to make that color be see-through. Once you’re in edit mode you can quickly go through the other colors in the model, and set them all to say 60%.