This confuses many new players....change the car "style" (last option in car dealer) to get to the in game cash versions.
As others stated....there is a level lock on igc cars, so you must level up to the required level, accumulate the required igc and have an enpty car slot in order to purchase the car.
Keep in mind that the game only provides 3 car slots for free....additional ones must be purchased for 500 speed boost each. Also, all cars purchased with boost from website and car dealer come with a car slot to put the car in (and the car slot remains if you sell the car)
Regarding game crashes....this can be a recurring problem for most players from time to time and is something we have gotten used to. If you are able to play other online games without issue, it is probably just NFSW or the game servers being tempermental.