You should probably at least consider Stencylpedia before posting really, really basic stuff like this...Anyway...
You can use blocks to create a script like this to spawn actors randomly:
(This would be an event for a scene)
-When created
Trigger [custom event]
-When [custom event] happens
Wait [pick random number 5-15] seconds
Create actor at [give it your own coordinates]
Trigger [custom event]
Also you should consider starting the car with 50 health and decrementing it by 1 until you reach zero and kill it. That's kinda how health works...