If this is going to be Your first robot, then my advice is think it through and pick something easier to start with, otherwise, most likely, it's going to be very frustrating experience. Building (semi)complex robots requires You to already understand what is required to achieve what You want. Asking about differences between Arduinos only suggests that You don't really know what features You are looking for and how to use them. Question about motors suggests that You don't really know what is normally used to actuate robots like that. If You are still interested in making one, follow the link below and read read read
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hexapod tutorial