First thing first, don't even care about destrucktor, it's wasted.
Second, survivor tank, timber beast or fire marshall ( I mean regularly fire marshall, not fire chief ), try to have 2 timber beast and your tanker is complete for now.
Next, support unit, only surgeon - sawbones is usefull, try to get sawbones to 50 is perfect, if you don't have, it's ok to upgrade surgeon to 50 too.
Gunfighter unit, gunslinger and commando is quite similar but commando is more worse with insane aim drift, so please get rid of her now. Scattershot and outlaw is good too but not enough, considering minigunner, he is monster. But important is grill unit, try to get at least one, three is done, you complete the game.
Range unit, best is sharpshooter with insane damage and perfect perk, dead shot can be use to mix with sharpshooter if there's a lot of little zombie, spec ops more like fun choose, machine gunner with right perk will take over grill unit.
You just need to get 2 timber beast, at least 1 gunsmith, is done, support unit doesn't matter, combine sharpshooter with dead shot, last 2 spot is your choose, it's doesn't matter since just with 2 tank and gunsmith will get the job done.