You're correct, in an original draft of Back to the Future, the time machine was a refrigerator. But in the story they would have had to load it onto a pickup truck before time traveling. The writers agreed to change the time machine into a car. During this time while the script was being written, John DeLorean was on trial and that's what gave them the idea to use one on the film. One more fun fact, Ford offered several million dollars to the studio if they used a Mustang as the time machine. The producers decided that it would not be in Doc Brown's character to drove a Mustang. Therefore, they declined the sponsorship deal with Ford and the DeLorean was kept as the iconic time machine.
