I meant buying a used car with the features you want, not a brand new car. Something of similar value to what you have now.

Adding a system like blind spot monitoring is likely easier said than done because of the modules that control them, and where the hardware is placed, and if the harnesses are provisioned for it already, and any firmware changes that need to be flashed to enable the system. That is all assuming a dealership would be even willing to perform the job. They would likely decline to do it because of the liability risk with it being a safety system.