@Jason Byrdsell welcome to BP! Atlanta is a good market if you can get in the right places. I have a guy who specialized in tax sale stuff there and likes the return he gets. If you are ever wholesaling in the Augusta area please feel free to reach out to me. As far as pre-foreclosure goes yes you are still signing the contract with the owner. Just make sure that you give yourself time to get the property taken care of before it goes to auction.
As far as the "zestimate" goes all Zillow is doing is taking property values within a certain range and averaging them together and coming up with that price. You can use that but finding good solid comps are always best. When presenting an offer to an seller having the zestimate, tax value, and comps are all good things to have. Are you looking to try to flip or B&H? If it is B&H I have a pretty cool "trashflow" analysis that I use for properties that dont have real good comps for them.
Feel free to reach out to me with any questions and best of luck!