The vacuum pump and the brakes.
Mostly all cars use a braking system with power brakes and this system needs to be left as it is in a electric converted vehicle. If the weight is bigger after the conversion because of the batteries, then the brake system should have at least the same braking capacity or even more.
Working brakes are very important with an EV since one way of emergency braking (engine compression, the one experienced when you downshift) is gone with an EV, you get no such thing from an electric motor, unless you have a regenerative braking system.
The difference between the braking system in a electric converted car and a petrol car is not the actual brakes, but the system on how the vacuum is generated to the brake booster. In a converted car this has to be done with a vacuum pump.
The vacuum pump is the same device as an air compressor, but the air valves are reversed so instead or throwing air it sucks it. In a standard car. De level of vacuum pressure needed for the brake booster is around 16” Hg to 18”hg (inches of mercury), that is about 54000 Pa (Pascals) or N/m2 (Newton/ m2), so this level of vacuum pressure is the one the vacuum pump should maintain.
There are electric vacuum pumps in some diesel cars as Volvo or vans whose diesel engine do not vacuum enough pressure for the brake booster. Those electric pumps work with 12V and they consume around 6 or 7 amps. You shouldn’t use the vacuum pumps designed for the door locking systems, as those do not provide enough vacuum for the braking system.
Sometimes a long braking push in the pedal ma use all vacuum pressure in the braking circuit, so the pump would need a few seconds to make the needed pressure level again. To avoid this situation a vacuum container (what a paradox, a container that holds emptiness lol) can be installed between the pump and the brake booster to keep some more time this level of pressure. Note that the bigger the container, the longer the braking capacity, that means, you can brake more times till the pump started again to generate the level of vacuum again.
Now, for the container to work, a vacuum switch needs to be installed to keep the vacuum pressure level above the given value, in our case around 15” Hg. Those sensors have a valve associated that opens or closes the brake circuit for the pump to maintain the vacuum.
This pump and components are very popular in conversion in the USA (SSBC pump), and the Hella vacuum pumps. I did order one for about 200€ plus 60€ of delivery. The surprise was when I received the package after paying some additional 90€ for customs tax, and saw the pump was Made in Spain.