This is gonna be another one that purists may want to skip.

My recommendation for the best experience, is to download the 3D model and hi-res texture packs from the addons page. Then remove the 3D models for all (wielded) weapons and enemies, instead opting to just use sprites. However, still retain the 3D models for pickups and other environmental objects.

This has become my preferred way to play SP Doom, Heretic, and Hexen.