"If you're looking for used, look for a KitchenAid made before the mid 80s. (1985, I think). The older ones were made by Hobart and had metal gears. They're indestructible."
The new ones do frequently have one plastic gear, which can break. This gear is specifically designed that way so that if the motor is in danger of getting overloaded, the gear breaks rather than the entire motor.
Mine did that last year and we finally fixed it. It's roughly a $5 part to replace and there are tutorials online about how to open up a KitchenAid and how to repair them. And, the only reason mine broke is because I was kneading a bit too much dough for my mixer and accidentally stuck a spoon in at the wrong time, giving it far too much resistance.
Although, I will also add that it took nearly a year to fix the mixer because I simply didn't find it to be a need. (I too make pretty much all of our baked goods.) We got it fixed partly because my MIL (who gifted it to us when we got married) threatened to buy us a new one and we didn't want her to waste that money. So whether you *need* a new mixer or not, only you can decide. It is nice to have, but if you've got a space problem in your kitchen anyway it might not be worth the storage space and the extra 5-10 mins/wk that you'll save on making bread.