Should be "Options" -> "Display Options" -> "OpenGL Preferences" -> [one of three options, I forget which] -> "Sector Lighting"; set it to "Software". I believe the settings for how you want the Spectres to be displayed should be hiding under "OpenGL Preferences" as well (the option name will have the word "Shadow" in it).
Also, I personally turn off all bilinear filtering - but leave mip-mapping and anisotropic filtering on. Something about Doom having chunky pixels feels right, but having Moiré effects (even if they're present in the original EXE), not so much.
Yeah, ZDoom has a crapload of options, most of which you'll probably never use or have no idea what to do with. Could be worse, I suppose.
Oh, one more mod for the pile:
Reelism is friggin' amazing. It's basically an arena mod, except every round has random modifiers (represented by numbered slot reels) applied to it, determining what monsters come out, what weapons you get, and one odd quirk on top of all that (low gravity, monsters explode when killed, super running speed, no friction, monsters run around confusedly instead of attacking, super firing speed, etc). There are even bonus modifiers if the numbers on the reels hit certain combos (for instance, 6-6-6 turns you into a Cyberdemon, 4-4-4 kills you instantly, 10-10-10 forces you to take a minute's break, etc). Then, every 5-10 rounds (depending on the gametype), you fight a boss monster, ranging from a ghost with plasma gun powers, another player, a Dog Pope, Hitler, or a wizard in a monster truck. It's unbridled chaos, and I love it.
GZDoom recommended. Doesn't work in Zandronum; it's based on too old a version of ZDoom.
Should work on recent versions of ZDoom (since those add support for 3D floors), but the framerate will likely be awful.