I'm considering purchasing a 2004 Kia Rio with.

"Auto trans doesn't work" can mean an awfully lot of different things. If it's computer-controlled, low system voltage is going to make it do strange things. Chrysler was the first manufacturer to develop a computer-controlled transmission, and I'm more familiar with their system. Once a problem is detected, regardless if it's due to a charging system problem or something in the transmission, it defaults to second gear and stays there so you can drive it slowly to a repair shop without needing a tow truck. That is called "limp mode". It will come out of limp mode when you turn off the ignition switch and restart the engine. In your case, I wouldn't get overly concerned with the transmission yet until the charging system is fixed. Then if a problem still exists, there may be a diagnostic fault code stored in the Transmission Computer that needs to be erased.

Check for fault codes first. When you disconnect the battery to replace the generator, any stored codes will be erased and that valuable information will be lost.