@Thuamaturge: You raise some interesting points, we'll have to test the different ideas to see which works best, but using a timer plus spawning zombies if it runs out might work.
Also you'll be glad to hear you can now pickup ammo in zombie mode. Dunno why I turned it off in the first place.
The zombie modes attack will be tweaked, making you able to attack faster but making the attack itself weaker.
Bleh now to get to work on the first two levels.