EVs use a reasonably common set of parts. There's always a motor to move the car, a controller to regulate the speed, and batteries to provide the power.

Roughly, the choice of batteries decides how fast your car will go, and the controller determines how quickly it will get there. Any motor capable of handling your battery and controller configuration will work, although some are more efficient, or more capable of withstanding abuse.

Before Nathan left for Reno, we knew we wanted a common configuration: 144 volts of sealed AGM batteries, a 500 amp or better controller, and the reasonably-common Advanced DC 9" motor. Even in a big car, we knew that would give us the performance we specified in The Requirements. We also wanted instrumentation, a heater, and other items that were technically non-essential, but required in a practical sense.

We found a guy in Texas who had advertised all the parts we needed for cheap. Not only did his kit have a 600 amp controller, it also included instrumentation, a heater, and assorted hardware we'd need (like relays). All for $2500.

My wife (rightly) refused to spend that kind of money without some guarantee of getting what we paid for. Unfortunately, the seller refused to enter into an escrow agreement, no matter how easy I made it. He called it "crazy". (Now, really. How long has escrow been around? And for just this kind of problem, too.) My wife pointed out that it would be crazy for us to spend that much money blindly. When the seller recommended I call City Hall to get recommendations, we decided to let the deal drop.

I mean, really, I can't trust him, so I'm going to trust a politician?!?

I'm not sure it has a good side.
Almost immediately after that deal broke up, we found a used EV just across the California border. It used the Curtis 1231C controller, which was "only" 500 amps. It also had the instrumentation, contactors, heater... literally everything we would need to convert our car. The only problem was that it had old, flooded batteries. But at only $1500, it left plenty in the budget. Just the parts were worth more than $3000. Nathan towed it home a week later.

This is a heater?
The only unexpected problem was its "heater". It turned out to be something like a hair dryer duct-taped to the dash. Ick. Still, at the price, I could afford to buy a ceramic heater to replace the old heater core.