I adore my Raspberries, but I would not use them for a doorbell. Other posters made great points regarding housing.
I actually use a regular IP camera (mini dome) just above the main door as a doorbell for our cats (OK, and for everybody else). I do not use BI or PC. Cameras are displayed 24/7 on a dedicated 7" Android tablet with TinyCam and when door camera sends motion alert, tablet rings.
If I do not want to be disturbed by the "doorbell" (or cats are not outside), I can turn the tablet sound off or set it to lowest level. Am using it this way for almost two months (this was my first camera), and the system works perfectly. Camera is set to trigger motion alert in advanced mode to avoid bugs and dust and similar. Light change is the only false alarm.
No CPU or any other problem. Tablet is on constant power and is held in place by sucky holder (think GPS or a phone in a car). Has UPS built in
Recordings are made separately and are not related to this use.
Actual doorbell button I may likely completely remove in the future.