Is it more expensive to add extras to a BMW when buying one used or buying it from the dealership?

I want to buy a used BMW 1-series but I want to add some extras. If I buy it from a third party and take it in, is it the same price to add the extras at the dealership or should I just buy the car used from the dealership?Is it more expensive to add extras to a BMW when buying one used or buying it from the dealership?Buy it used from the dealership, you don't no what you're getting with a third party and it is hassle free to talk to a dealer about extras and if it is a good dealership they actually know what they are talking about and install/upgrade your vehicle appropriately with true german parts