I have wanted to try the D20, so I think in today's game, I'll try your design.
The Zombies don't do much in melee but then they are supposed to be a "hore" and over whelm you with shear Undead goodness. :)
Shooting Hits is just an + to your target rep to see if you hit and combat is based on the Impact of the weapon. So you basically have to Hit, and then check damage or impact. I use the SMG mostly, so I get up to 3 die per shot. It is a little confusing to go from rolling high to needing to get 3 shots BELOW my weapons Impact for the damage.. I am not sure if thatt is in the original ATZ rules, but I am looking at it for future expansions.
RebelMike