Re: Commuter in AR
Yes, you do want to mount the motor as low as possible. But if you want to shift your bike that does not mean remove your front derailleur. I think once you get your kit you'll realize its not even a issue.
I never buy motors off Ebay so I cannot say you are being unreasonable. But I would avoid the chrome engines they are notorious for seizing up. They just don't dissipate the heat like they should and run too hot.
When I worked for a local motorized bicycle shop, every single one of our chrome motors seized up. The shop lost a lot of money on those motors.