My idea of a minimalist HUD, strictly speaking, would fit EVERYTHING into where that current HP/Armor/Weight is.
For reference, the HP and Armor bar that I've made is as tall as this sentence and reaches stretches from the F in "For" to the I in "I've". Right below that is a binary weight bar that is not unlike the one in the store that lists from 1-15 blocks being full. I've also designed 27-pixel tall class representations (approximately the size of 2 of these lines) with individualized dot representations for stars.
Ammo checking doesn't work so well by looking at the magazine, because each class gets the expanding ammo pool. Look at the MP7, which goes from 20 in a short mag, to 40 in a short mag. My idea for ammo would simply be displayed as TOTAL AMMO POOL (stuff not in your gun, out of 100%) and as you reload, it drains from the bar. Would represent your ammo container better because otherwise, the grenade launchers would have a huge amount of magazines, as would a stock MP7.
Basically, we have this on our HUDs standard:
HP - vital
Armor - vital
Weight - non-vital, but helpful
Class and Rank - non-vital, but useful for showing your player status in screenshots.
Flashlight Charge - non-vital, can be either made into a slim bar or removed entirely.
Ammo count - Replace numbers with striped bar for ammo count and solid bar for pool count.
Grenade count - can be represented as multiple icons.
Money count - It should be hidden unless you get cash (shown as a +$XX instead of as a total), throw cash, or buy stuff.
Trader Compass - looks silly and is somewhat inaccurate. Should be replaced with a slimline horizontal compass that points you in right direction, as well as some sort of functionality for vertical height and distance.
ZEDs Left/Time Left/Wave Count - Wave count can be seen in the score sheet, but I guess it can be displayed as skull icons with (/) crossed through for each wave completed, current/next wave should be glowing, pat gets his own icon for 11. Zombie count isn't normally applicable for the game. Simply audio cues is sufficient, and would make the game more hectic. Time left for trader should appear here, but without the silly background.
The HUD breaks the atmosphere as it currently is.