There are hundreds of thousands of cars out there waiting to be converted. The best cars to be converted to electric are small old cars. The older is de car, the less electronic components it has and the easier will be the conversion.
As the batteries are still the big issue in a conversion project, regarding the weight and the price, the only way to minimize this aspect is by installing the less battery modules possible, and this can only be achieve nowadays by using a donor car that needs little electric power to drive, therefore it will need few batteries to run.
Another advantage in this philosophy is financial cost in the project. An old car will always cost you less than the equivalent two years old car. The Mini may be the exception in this case, but hey, it is a Mini.
Here are some examples or perfect old cars easily convertible, and examples of their prices in the second hand market.