I'm planning on specializing in aeration in my local area. However I'm slightly concerned about trashing the machine on rocks. I'm about 90% sure the 850 HD is the way to go however I would consider the other Plugr models if I can find a decent used unit. How often have you had to replace tines? Bent the crank assembly? Have you ever broken the machine? **Side Note** Is the hydro drive on the 850 a single speed or variable? Have you used the step saver? Is it worth it? Also what did you pay for your unit? $ What is the better engine - Honda or Brigs?