Well I for one welcome our zombie overlords
anyway back to zombie smiteing
Sledge hammer too heavy for sustainable zombiecide, plus to use one effectivly require total comitment to the attack.
A good quality hardwood combines wackability and light weight
I would go for a one handed weopon, leaving the other free for defensive strategies
Blunt a good machete and it fills every category you've got there. So how is it not the best?
Also, you're not taking into account that even a blunt machete would work well, light weight equates to faster swing speed and less fatigue, thin but sturdy profile equals force distributed through smaller surface area. It balances out nicely.
If you were ordering cold steel magnum Kukrhi machetes, you could buy 5 for about £100, so stocking up wouldnt totally bankrupt you, besides, thats a high cost quality machete. Cheaper ones are also available. Maintenence is little more than oil and a good grit stone.
I've been using one of these at work and they could easily remove someone's head.
It has obvious draw backs like starting from cold and reliance on fuel and noise but if you knew you were going to be in the open you could make very short work of an awful lot of zombies.