As stated above you can't do crop spraying on a PPL.
It's not for the faint hearted and it is imperative that you get good training. You'll need it for the flying and for the handling of the chemicals. Most start off in the smaller piston taildraggers and then move onto the turbines. The good guys spray at night in Oz.
There's good money to be had crop spraying. The top guys in Australia earn about $A60-100,000 a year.