Are your thinking about making website? OR you are worried about your old website?. You are not late, just get started with responsive design website. Think about coming generation and upgrade your thought and ideas :).
If you want your website presence in major search engine then you must undergo SEO for websites. Responsive Design helps to promote your website in different search engine like google,bing etc.
Good Coding make some sense for better perfomance and good for Search Engine Optimization too. I love clean and well structure coding and other programmers expect the same too.
Must have project documentation
Extra Cost for additional changes