By and large Asset Forge was made for people like you, so fear not.
Chances are you won't have to make or import custom 3D blocks. There are some incredible examples made 100% with default blocks arranged in creative ways. If you do find yourself needing a custom block there may be something very like it out there in a custom pack by the time you need it, and if that doesn't work there are tons of tutorials out there for 3D modeling software. What's "easy" for one person may not be easy for another but there are entire Youtube channels dedicated to teaching Blender or Sketchup or Rhino or... you get the idea.
No, you can not import 2D objects. There's a somewhat hacky way of importing a PNG as a texture on a material but that's out of scope for this. If you buy the program and really need this functionality and Kenney hasn't added custom textures at that point ask here again and I'll show you how to do that.
Custom textures are in the works but as of now (V1.1) there is not a direct way through the Asset Forge interface. Soon
Right now Asset Forge essentially takes a picture of your 3D asset from any number of angles and exports that as an PNG for your 2D asset, so if your asset is not built in a pixel art style your PNG will not be.