I don't see any reason why any common biowarfare agent couldn't be dispersed from a suitably modified cropduster aircraft. To my knowledge, bioagents are powders or liquids which, to be delivered, need to be finely dispersed in the air so that the intended victims inhale them.
I have no clue about the area that a cropduster could cover in a flight. That would depend on the wind, the aircraft tank size, the nozzle spreading efficiency, the required concentration of agent, the altitude at which it flew...
Yes. Weather plas a strong role in the effectiveness of chemical and biological attacks. As the most simple example, high winds will disperse & dilute the agent to be below the necessary concentration relatively quickly.
Precautions against chemical and biological terrorist attack are the much-mocked-and-derided duct tape and plastic sheeting. No, really. Sealing a space off from the outside and sheltering there is the safest thing to do. Be sure to have plenty of nonperishable food, and at least one gallon of fresh water in sealed container per person per day. Try to have at least three days' supply, and a week is ideal. Your chemical or biological shelter should be at the highest part of the house, because chem and bio agents are heavier than air and will sink. (Note that weather and radiological shelters need to be underground, in contrast.)
In the case that one is caught outside in an attack, the first thing to do is to not panic. Panicking only sets the body's metabolism in high gear, drawing in more from the outside and forcing a higher exposure. Go uphill, upstream, and upwind from the contaminated area. If in a car, roll up all windows and turn off the vents and fan. Wash with soap and plenty of water; a bleach solution can be used as a neutralizing rinse. Use your NOAA Weather Radio to get alerts; the local broadcast media outlets will also have information.
Most of your concerns are addressed in the book Are You Ready?, available as a PDF for free download, or they'll mail it to you for free.
Very helpful information. I would of thought to go to the basement. It's good to know that the chemical would be heavier than air and sink. It's tips like these, easily remembered, that are the most helpful.