It sounds like you already know how to turn a layer into a sphere. An accurate image map to use for a pumpkin would be a Mercator Projection. This is difficult to do in Photoshop from a still image. You'll need to have shots from every side and then put it together. You could set up a rig to rotate the pumpkin as it moves and then use the Panorama feature from your smart phone. You could just make the solid with some stripes painted on it with AE's Brush tool. A perfectly spherical pumpkin would be the result because there are no distortion or bump mapping controls in CC Sphere. You would be better off modeling the pumpkin in C4D or Blender and creating a bump or displacement map to give the pumpkin a more organic shape. A quick search of Google brought up this Pumpkin texture map.
Throw in some duplicates, CC Cylinder for a stem, a few paint strokes and some blend modes and Trapcode shine and you get something like this.
Kind of Cartoony but OK.... Here's the project: JackOLantern.zip
Which version are you using? C4D Lte was introduced in CC and that will give you basic 3D capabilities. CS6 and Ray-traced rendering will not give you what you need to make a pumpkin.
Blender, an open source and free 3D app will give you everything you need to build a very realistic pumpkin or any other 3D object and now includes some very good integration with After Effects. The example I posted could be created with any version of AE that has CC Sphere or even in AE 3.1 (a zillion years ago) with a 3rd party Sphere plug-in.
I wonder if the latest version of After Effects would be useful? I've
heard its true 3d.
AE is NOT true 3D, but C4D Lite which comes with AE is, and it ought to be able to make a pumpkin. Which you can then use in AE.