what would you say is the best resource for learning about building one from scratch in JS?

I followed Inigo Quilez's article on terrain marching:

http://iquilezles.org/www/articles/terrainmarching/terrainma...

Be sure to browse his other articles:

http://iquilezles.org/www/index.htm

and his awesome video tutorials (which tend to be more about the math than the code):

https://www.youtube.com/user/mari1234mari

Enjoy!

The simplest techniques for 3D rendering without relying on a GPU and WebGL are indeed (2.5D) raycasting engines (such as this Voxel Space engine). A short google search reveals for example:

http://www.playfuljs.com/a-first-person-engine-in-265-lines/

Also read the readme file of this Voxel Space engine. Maybe this is already enough to understand the principle.

Loading .obj files is pretty easy, and basic triangle rasterization isn't too hard either. Still probably a bit harder than height maps.