It seems like you already have a leg up 99.9% of people starting out, both because you already have ready, willing, and able buyers for your product, and your father appears to be a tremendous informational and financial resource.
As a 16 year old, however, I don't believe you can enter into the necessary contracts to secure insurance coverage, or apply for an EID # (needed to purchase parts wholesale in most cases). I could be wrong about that, but I believe you will need a partner or trustee.
I'm not aware of any certifications you would need to build and sell computers, although, things vary from state to state.
As far as warranties are concerned, it is a personal preference for how long you are willing to support the products you build. Legally, I believe you are required to provide some type of warranty unless you are explicitly sell products AS-IS, and the laws governing warranty structure also vary from state to state.
I've never purchased wholesale computer components; I just buy my parts from NewEgg.com, so I can't really offer anything there. However, stay away from anyone that doesn't require an EID #. I think some of the old threads on this forum talk about WorldWideBrands or something like that, it may be a starting point.
Danny
Edit: PS. Can you post the specs for your machines. I've been thinking about picking up a high end gaming machine.
