Help with Selection of Motor AC or DC

Hi,

    I have a bit of knowledge about electronics but learning more while working on this project.

   I am working on a project where I am designing a electronic deivce (close to a robot), which can be used for working on agricultural fields. The size of the device is planned to be 30cm width and 30cm of length. The device that I am planning to make is much closer to the image in the following link : 

http://www.odt.co.nz/news/farming/280043/mars-rover-inspires-pasture-based-robot

But a bit smaller in size and also the device would have rotating blades in the front of the device which will be designed to scrub the soil to remove weeds. I am using Atmega328P as my Microcontroller, and planning to use 6 motor, where 4 for the wheel and 2 for helping the blades to rotate in the front to scrub the soil.

  I need some advice with the selection of the motor that I can go for. I have done a bit of research for motors and I am new to motors too. With my understanding I have the following details.

What I am looking for is Reliability, Low Cost, Less Maintenance, Longevity, Good Torque (for the motor used for blades), Moderate Torque (for motors used as wheels), Moderate Speed, Should also be used in hot and hazard environment where there could also be possibility of little water.

 

if possible, Good Starting Torque would also be appreciated for the motors used for Blades.

 

Voltage Range : 12V to 30V (More voltage can also be considered, if the motor is reliable)

after little research I concluded with the following:

1. DC Permanent Magnet Motor

2. AC Induction Motor (e.g. Squirrel Cage)

 

DC Permanent Magnet Motor: 

These motor are expensive, not sure about the longitivity but provides enough torque for voltage supplied (e.g. 30V)

 

AC Induction Motor (Squirrel Cage):

These motor are less cost, low maintenance, can be used in hazard environment, can be used in hot and rough environments. But I was not able to find any motor that is small enough to fit in my project. Also require a control or starter circuit for this motor.

The above mentioned are my understandings, If anybody have any other suggestions or other alternatives that I can used. Please let know.

 

Many Thanks