When you want to build your own electric vehicle there are basicly three approaches that can be followed. The first is to use an existing EV and refit it with the parts that you have.
The second is to use an existing ICE vehicle, take out the ICE drive train and replace it with the electric drive train. The third is to start from scratch and have all the freedom to design the vehicle you want.
By far the easiest is to get an existing (electric) vehicle and retrofit it so that it becomes an electric vehicle. There are a few things you need to consider and be aware of, but it gives you a head start and you do not need to worry about welding a chasis, getting comfortable chairs and other systems you have in a car. The harder route to go would be to start from scratch; this gives you a lot of freedom in your configuration. From designing the drive train, the exterior and the interior, you have a lot to thing of. Suspension, power steering, Airco, etc.
Recycling: Getting a used car and fitting an electric drivetrain in it.
Regardless of the route you want to pick, the first step is what do you want to use the vehicle for, mainly. It is no use to design and build a compact two-seater sportscar if you plan on moving large loads with it. You get the idea. Once you have determined what the main purpose of the vehicle will be, the global size of your vehicle is set. Either it will be a small compact chasis that you need, or a larger one depending on your goals.
Since the second hand market for electric vehicles is very small still (Who would want to part with such a nice machine anyway?), most people will not go for option 1 [!!]. When looking for an old(er) vehicle to convert into an electric vehicle, the state of the engine is not going to matter a lot. But things as the shocks, the gearbox and the looks of a car can still make a difference between an easy or cheaper project versus a hard, time consuming money sink. All the parts that you do not need to fix or upgrade are a plus.
The electric drivetrain allows for totally new designs, like the Aptera here.
n the case you want to start from scratch and have decided what the main purpose of your vehicle will be, you can 'simply' start designing a chasis to fit your needs. The requirements in this case being for example; being able to carry x persons and/or y amount of cargo in sunny/winter conditions. In this case you might also want to take a look at what drive train you want to implement, as the electric drive train allows for a lot more freedom in your design than the conventional drive train allows. For example, there are in-wheel motors, which eliminate the space used by a motor, a gearbox and the interconnecting parts. Currently perhaps a bit expensive, but they sure open up a lot of possibilities in your design!