A Bull Terrier is a Bull Terrier and has every right to alert his/her family upon approach of anyone or anything unusual. These are highly dedicated family dogs and will show it if need be. That is her turf and she is going to protect it. You’ll find this type of behavior helpful down the road. I just hope you never have to see her go into full protection mode cause then you may want to put her in a shelter because of your own fears. These dogs are very loving with strangers as long as it’s done in the right manner. Their tenacity is second to none. Do your homework well and if you still have second thoughts give the dog to someone who is well educated with this breed so she does not find herself in a shelter by ignorance of ownership.
I am 58 years old and this is the only type of dog I have ever had since I was eight years old and then we used them as working dogs to pull apple carts in from the orchards. This was before most even knew the breed existed.
Think it through before you act.