Your first problem sounds like a defective CCD imager. If your camera was an S300 Canon should fix this for you for free, including free shipping both ways. Please check the following two links for more info:
http://camerarepair.blogspot.com/2007/11/canon-digital-cameras-showing-black.html
http://www.usa.canon.com/consumer/controller?act=PgComSmModDisplayAct&fcategoryid=225&modelid=13390&keycode=2112&id=29819
Applicable cameras include:
A40, A60, A70, A75, A80, A85, A95, A300, A310, S1 IS, S60, S200, S230, S330, S400, S410, S500, SD100, SD110
Unfortunately, you mention that your camera is an SD300. That model camera is not currently on the recall list. But you might want to call Canon anyways. You might get lucky as the problem sure sounds familiar to the recall (excuse me, advisory hehehe).
As for the SD500 with the E18 error, try
http://www.e18error.com/repair.html