Awesome, but....
I recommend difficulty increases as relevant with the currently purchased weapons.
For instance, when you get the jackhammer everything should get much harder by increasing the speed and attack strength of all zombies and increasing spawn rate drastically, or perhaps it would be better to limit the high-powered weapons more.
Alternatively, implement a system of limited ammo such that you must purchase ammo, making expensive weapons harder to maintain....
Overall, it's not exactly a new concept, but it is rather well executed.