$contents = fread ($handle, filesize ($filename));
(cut...)
mail($SendEMail, $MessageSubject, $contents, $mailheaders);
If this part "Dear $_GET['ToName']," is in the read file, than it is read into $contents and directly put into the mail. At no place are you asking PHP to replace the variable name with its value.
You could place a simple code in the file like ##ToName##, then do a
$contents=str_replace('##ToName##',$_GET['ToName'], $contents);
To make it more flexible than having to write a line for everything you could have in the file you could have a look at how templates work. Or dig into regular expressions.
Chris H.