New Weapon: 1300 range, 30k gold.

[GooglyBoogly]

I like the idea of penalty against buildings but can this be done with a single weapon, without creating 2 weapons for it?

as a constant effect, no - you couldn't have a weapon that can hit anything, but with a penalty to only building damage, as a switch however, it has already been done
Switch would probably be between "cannot target buildings" and the normal anything goes.

Constantly clicking on an item to change modes is rather annoying.

that is the point, if you want to be able to hit buildings with no chance of taking any damage in return, that's gotta cost ya. Besides, you probably wouldn't constantly click, since there is a (normally) 3 sec switching time between modes, during which the weapon doesn't fire at all.