echo esc_url( get_permalink($post->ID) + complete the ID’s url

The best approach, from my point of view, is to use add_query_arg() function:

$only_conent_url = add_query_arg( array( 'content-only' => 1 ), get_permalink( $post->ID ) );

And then you can escape it if you need:

esc_url( $only_conent_url );

add_query_arg() takes the URL passed as second paramenter and append a properly formatted query string built from the key=>value pairs contained in the array passed as first argument.