5 hours ago, Plotnus said:

I feel the asset that really gets me is animation and I have a feeling that this is a hard one to learn.
 

Yes, that one is the hardest it seems for everybody.  One thing that compounds it is trying to mix and match animations with models.  If you are gonna try out Blender, here's the list of things I would do first.

1) Learn Mesh work first.  Master exporting to .fbx format from Blender.  This is key to having models AND animations appear and function correctly in Unity.  Just use basic shapes and scribbles to test that your meshes are importing into Unity without any weird rotations or transformations.

2) Figure out how to work with the Armature setups and Pose Mode in blender.  Don't worry about attaching a mesh to it just yet.  Open blender and delete everything, add in an Armature and go about some tutorials on animation from there.  Any animations you make using a Humanoid(metarig) Armature in Blender(If exported correctly) should work with the Standard Assets ThirdPersonController setup, or any other standard player controller/Animator setup for Unity.

3) Combine the two..  Make a simple prototype humanoid character, add an armature.  This is called Rigging. 

Like I do everything, I always tried to tackle all 3 subjects at once, and in Blender that doesn't work well.  There's just too much to the program to think that you can digest it all in one sitting..  Break it down into small parts and it will make WAY more sense. 

And bookmark the links you find with all the handy hotkeys and basic function tutorials, you may have to return to them a few times..

Good Luck!

This particular line you said: 

"One idea is to stitch something together using primitives. I'm really not sure how to go about this one."

Always always do things first with basic very simple assets like primitives. It's one of the number 1 mistakes during prototyping or any beginning phase. Nail it down with very basic assets. Simple forms, simple primary or gradient textures/gradients. Any color used at this stage should be purely colorcoding for game play or identifying type of asset. Animation should be only the most needed key animations. Most of them can be done in engine even or procedural generated. Resist the urge to go too beautiful early on. If you can make it feel right with primitive, simple shaders, basic animations, you will definitely be able to make it feel right with more final art. When prototyping game-play many aspects don't function at start and for a long time they won't as you are figuring it out and trying things and too often people try to "fix" it with beautiful art. 
I'd even say it's one if not the main reasons why so many projects that could have succeeded fail. 

I'll throw out option four which has not been mentioned, learn more about procedural generation and take your generic assets and kick them up a notch with shaders. 

I would encourage you to give Adobe Fuse + Mixamo a try. Fuse lets you put together (not model from scratch, so you don't need to be an artist) 3D models from existing assets and then you can export them to Mixamo, where they are automatically rigged for you. After that you can apply animations from Mixamo's library to them. Their animations library is quite extensive, it has categories like action, combat, sports, etc. You can tweak animations, i.e. make them animate faster or slower, etc, and then export into FBX (and other formats) for Unity. I use those in my game and they have been a lifesaver. Both Fuse and Mixamo are free right now.

Also, like other people mentioned, OpenGameArt.org is a great resource for 2D, 3D, textures, and even sound effects.

You can also make your own assets. People have already mentioned Blender for 3D models and animations. One thing I'd like to add is you can use XBox Kinnect for low cost motion capture for more realistic animations. For 2D graphics you can use GIMP 2, it's free.

And finally, you can try and find an artist to work with. If there is a game development association or an IGDA chapter in your area, you can join and attend a session or two, which would be valuable learning experience and there is a high chance you can meet an artist there that would work with you.