I see somebody has an ad for an Autohelm 5000 autopilot for sale. Does anybody know anything about this model? I assume it's more heavy duty than the 4000 or 3000. Would it be appropriate for one of our boats? Also, any ideas on what would be a good price? (I'm assuming it's used, so I'm thinking not more than $300 or so since I don't think parts are available. What do you think?) Finally, are the parts interchangeable with another model Autohelm? For instance, could I replace my motor (and ring) with the 5000 and keep the rest of my autohelm?