Any car can be converted. It's a function of driving requirements vs. investment. In general, it is nearly always more economical to purchase a modern electric car with all the creature comforts and technology ailable. But, what's the fun in that?
Ecologically and emotionally converting or re-purposing an existing vehicle is a sound decision. It takes a tremendous amount of energy to produce an automobile. If the shell is in good condition, then converting it is environmentally net positive.
So, what are the best cars to convert? It's a personal preference. Just remember that you will be driving it for a long time, so choose wisely.